GERMANY’S ROLE in conventional piston engine development during the Second World War is often overshadowed by that more showy upstart, the turbojet. But given Sir Roy Fedden’s considerable contribution to aero engines during the inter-war years, you can be sure that he was not going to give the German piston engine builders a miss. Of the main engine companies in Germany, Junkers, BMW and Daimler-Benz were considered to be the most important, certainly in Fedden’s opinion, as they had continued with piston engine development throughout the war and had supplied engines for all of the first line fighter and bomber aircraft of the Luftwaffe. At the height of wartime production, these three companies had employed between 150,000 and 170,000 people and, with the help of the shadow firms, they were delivering in the order of 6,000 to 7,000 engines every month. The Allied bombing campaign, however, dented these prodigious output figures, reducing them to about one third, as Fedden confirmed in conversation with a number of the German engineers encountered during the 1945 mission:
Most people commented on the efficiency of our ‘communications’ bombing, which they said was far more disastrous from an engine output point of view than actual bombing of works. Few works could not be repaired or dispersed in a comparatively short time, but the bombing of rail heads, trains, lorries, roads, etc, absolutely paralysed production and efficiency.
JUNKERS FLUGZEUGWERKE, DESSAU
On Monday 18 June members of the Fedden Mission travelled by road from Kothen to Dessau to interview technicians at the Junkers Flugzeugwerke in Dessau. All new Junkers aircraft, right back to the Ju 60 single-engined airliner of 1932, had first flown from here and by the outbreak of war the works had expanded to cover a massive area. Even so, actual aeroplane production was increasingly transferred to other sites in order to meet the high demand for Junkers’ flagship aircraft such as the Ju 87 Stuka – short for Sturzkampfflugzeug, meaning ‘dive bomber’ – and the twin-engined Ju 88 light bomber. Inevitably the conspicuous complex of buildings at Dessau attracted the attention of the Allied bombers and in one particular attack, on 30 May 1944, the works were virtually wiped out. Fedden reports: ‘Damage was so extensive that little of value could be seen in a quick look round, which was all that was possible with the imminent approach of the Russians.’
The engine company, Junkers Motoren, otherwise known as Jumo, had separated from the aircraft part of the company in 1923. Their primary piston engine of the Second World War was the Jumo 211, widely considered as a rival to Daimler-Benz’s DB 601. German aero engines were designated by a three-digit number with the first digit identifying the manufacturer: 1 or 8 for BMW, 2 for Junkers, 3 for Bramo, 4 for Argus, 5 for Hirth, 6 for Daimler-Benz, and 7 for Bückner or Klöckner-Humbolt-Deutz. The following two numbers denote the individual engine series.
The Jumo 211 was an inverted V-12 in-line liquid-cooled engine giving between 986hp on the earlier A model and up to 1,479hp on the final P version. Total production of the 211 series was over 6,000 engines and in addition to the Junkers Ju 87 and Ju 88 it powered, among others, the Heinkel He 111E, H and Z, the Ju 90 and the Ju 52.
The Jumo 213 grew directly out of the 211 design, with the open cycle water cooling replaced by a pressurised cooling system that emulated the DB 601. This reduced the amount of water coolant required, making the engine both smaller and lighter. Other improvements boosted the power by 500hp, resulting in the Jumo 213 in its various derivatives becoming one of the most sought-after piston engines towards the end of the war. Over 9,000 Jumo 213 engines were built. They were fitted to a long list of Luftwaffe aircraft including the redoubtable Heinkel He 111 bomber, the Junkers Ju 88, Ju 188 and Ju 388 twin-engine multi-role/bombers, the single-engined Focke-Wulf Fw 190D and Ta 152 fighters, the Ta 154 night-fighter, and also the Messerschmitt Me 209(II) which was a proposed beefed-up version of the highly successful Bf 109. Messerschmitt had wanted the DB 603 engine for the Me 209(II), but as this was in short supply the Jumo 213 was substituted instead. The prototype made its maiden flight in November 1943, but in the event only four Me 219(II)s were completed before the project was cancelled in 1944. After the war the Jumo 213 also appeared on some versions of the Nord Noroit, a French-built flying boat.
Early Luft Hansa Ju 52/3mho featuring Jumo 205 air-cooled six-cylinder inline diesels. Later versions were fitted with BMW 132 radials based on the Pratt & Whitney Hornet.
In their discussions with Dr A. Scheibe, the technical director in charge of Jumo piston engine development, Fedden’s team learnt that out of a large number of Jumo 213 projects, three definitive series models could be singled out for attention: the 213A, the major production version with one-stage two-speed supercharger; the 213E with intercooled supercharger, basically a high-altitude version of the 213A; and the projected 213J, two-stage three-speed, but with a different ratio and four valves instead of three per cylinder. Its weight with intercooler was 2,325lb (1,055kg) and the power output was 2,350hp for take-off at 3,700rpm. The 213J had not entered production by the time the war ended. The mission team later came across several brand new Jumo 213-Bs, still crated up, in the Junkers factory at Magdeburg. Other Jumo piston engine projects, such as the ill-fated Jumo 222 with twenty-four cylinders, and the diesel developments, had stagnated in the latter stages of the war with all effort being concentrated on the 213 series.
On 1 July 1945, Dessau was officially handed over to the Soviet Military administration and in 1946 Dr Scheibe and his team went to the USSR where they continued with the development of the Ju 287 (see Chapter 3).
BMW WORKS, MUNICH
In contrast to Junkers’ bomb-damaged plant at Dessau, the British team managed several trips to inspect the BMW works in Munich, mostly between Wednesday 20 June and Sunday 24 June 1945, and again during a supplementary visit to Germany in July. This gave them the opportunity to speak to a number of the BMW engineers. In charge of piston engine development at BMW Munich was Dr Amman, aided by his chief assistant Mr Willich, and Mr Sachse who had been the senior engineer up to 1942 before he started his own works at Kempten, Bavaria, for the manufacture of automatic controls. The chief engineer and technical director in overall charge of BMW’s jet, piston and rocket development was Buno Bruckmann, an old friend of Fedden’s and a man very familiar with the Bristol Jupiter engines which were being built under licence in Germany before the war. (Along with the former managing director Dr Popp, Bruckmann was imprisoned by the American authorities shortly after the mission’s first visit to Munich. Fedden noted, ‘We gather the grounds are subversive activities, but as far as we know no charges have been preferred.’)
Piston engine development at Munich was focussed on three main objectives: Developing the 801 up to the limit, getting the eighteen-cylinder 802 engine into production, and continuing development of the big twenty-eight-cylinder 803 engine.
The BMW 801 was an air-cooled radial engine with the cylinders arranged in two rows of seven. A staggering 61,000 BMW 801 engines were built, the largest number of any German wartime radial engines. It was provided with a single-stage centrifugal supercharger with two automatically changing speeds, and a direct fuel injection system. The 801A, for example, weighed 2,669lb (1,213kg). Best known as the power-plant for the Focke-Wulf Fw 190, the BMW 801 was also fitted to the Blohm und Voss lop-sided BV 141 reconnaissance aircraft, the prototype BV 144 which was intended as an advanced post-war airliner, the Dornier Do 217 twin-engine bomber, a whole family of Junkers including the Ju 88, Ju 188, Ju 288 and Ju 388, as well as several candidates for the Amerikabomber project. These included the Heinkel He 277, which was a four-engine heavy-bomber derived from the fire-prone He 177 but never completed, the Ju 290 four-engined heavy-bomber and its offshoot, the six-engined Ju 390, plus the Messerschmitt Me 264 previously mentioned.
Cover of Der Adler for November 1941 shows Ju 88 fuselages nearing completion. Around 16,000 of the multi-role twin-engine aircraft were built.
The 801 had not been in production when the war started, but with pressure coming from the RLM development was accelerated under the direction of Sachse. This included testing the complete power plant at LFA Völkenrode. Through extensive wind tunnel exploration it was thought possible to make considerable improvements regarding the drag of the piston engines, and a reduction in drag to the extent of 150 to 200hp was considered a definite possibility on the 801 engine installation for the Focke-Wulf Fw 190. The wind tunnel testing also enabled the engineers to maximise the use of positive air pressure built up in the cowling in front of the engine to cool the cylinders, cylinder-heads, crankcase and so on.
The Junkers engine plant at Dessau was conspicuous from the air and is shown during an attack by Allied bombers on 16 August 1944. (NARA)
The 801-1 was the turbo-blower version. The blower was mounted high up behind the engine, with its axis sloping forward at about 30° from the vertical. The turbine wheel had hollow steel blades, similar to those in the jet engines. Fedden saw a number of these engines abandoned and left in the open on Kassel airfield. The final production version of the 801 series was the 801-E which had a pressure die-cast hydronalium cylinder head, chromium-plated cylinder barrels and exhaust valves, and stronger pistons. The 801-E, which had not entered production, incorporated a strengthened crankshaft necessary for power in excess of 2,000bhp. It was anticipated that this engine would be rated at 2,600bhp with methanol-water injection.
The BMW 802 was an eighteen-cylinder engine of ‘interesting and unorthodox design’, as Fedden put it, attributed to Dr Sachse. A two-stage three-speed blower gave a rated altitude of 32,000ft (10,000m) and the maximum speed was 2,700rpm. The engine weight was 3,380lb (1,530kg). Emphasis had been placed on the manufacture of the complete power plant with clean air flow throughout. The unusual front and rear valve position was extended to give the shortest and straightest possible air and exhaust passage, and the positioning of the blower on the front was adopted for the same reason. Air entering the power plant was compressed by a fan and stator ring, then taken off by the blower and dealt with separately. The remaining air divided into three parts passing through the intercooler boxes, the front row cylinder baffles and the rear cylinder baffles. These three streams of air then joined up behind the rear cylinders and were ejected by the exhaust gasses. With this design the ejector thrust and mechanical work to the fan just balances the total internal drag. Several of these engines were made in 1942, but development over the remaining few years of the war was dropped in favour of the 801. Fedden observed:
Post-war scrapyard heaped with unwanted German aero engines. (NARA)
Dr Amman himself thought the 802 was a good engine, and that if more power was to be the goal, then they would have done better to have gone to the 802, rather than flog the 801, which was now pushed to the limit. He thought that 3,000bhp would have been obtained fairly easily.
Fedden personally considered the 802 to be one of the most interesting piston engines seen in Germany and well worth following up.
The BMW 803 was a project put forward by the Siemens part of the BMW organisation in Berlin, but it was not well received by the Munich team and was still languishing in the very early stages of development by the war’s end. The 803 was described by Jane’s All the World’s Aircraft 1945/6as having ‘the appearance of two fourteen-cylinder engines joined together’, which is pretty much what it was. Designed by Dr Spiegel, it consisted of two fourteen-cylinder units which ran independently but coaxially, the idea being that one unit could be cut out if required. (In tests it was subsequently found that the bearings of the idling engine tended to break up under these conditions.) The two coupled units drove twin coaxial propellers on the front end and a pair of blowers side by side at the back. Each fourteen-cylinder unit consisted of seven radially disposed twin in-line liquid-cooled units, which were detachable with the barrels and valve gear. There were two valves per cylinder, operated by an overhead camshaft, each driven by a bevel-geared radial shaft. The capacity of the whole twenty-eight cylinders was 83.6 litres and 3,700bhp was anticipated for take-off at 2,700rpm. The prototype weighed 7,714lb (3,500kg) complete with propellers, but this was more heavily built than the production versions would have been. Fedden understood that only about twelve 803s had been built and the team saw examples in pieces at the Munich works, but they remained unimpressed by the engine and described it as a ‘pedantic compromise’. The Mission Report stated:
Its layout and design appeared clumsy and rather indifferent, and gave the impression of having been designed by one with an air-cooled radial mentality, yet without the courage of his convictions.
DAIMLER-BENZ HEADQUARTERS, UNTERTÜRKHEIM
There hadn’t been enough time for the Fedden Mission to visit the Daimler-Benz works during their first visit to Germany, but a group was despatched to the Untertürkheim on the second, supplementary trip in July. Located on the edge of Stuttgart and right beside the main railway line, Untertürkheim remains the headquarters of Daimler AG to this day, but back in the spring of 1945 the works presented a scene of utter devastation. Due to the level of bombing the production had been almost entirely stopped between 1943 and 1944, although the aero-engine factory had continued to function as Daimler-Benz’s engine development centre.
Heinkel He 111 being stripped by American personnel. (NARA)
Prior to leaving for Germany Fedden had been briefed on various Daimler-Benz developments with information mostly obtained from the interrogation of German personnel held in London. However, he was left with the impression that some of this information did not fall in line with what they were being told by the general manager, W. Haspel, or the chief designer, Dr E. Schmidt, over in Untertürkheim.
The Daimler-Benz DB 603, a liquid-cooled in-line twelve-cylinder inverted V-12, was a successor to the earlier DB 600 and DB 601 engines. The latter was basically a redesign of the DB 600 incorporating direct fuel injection and improved supercharging capacity. The racing version of the DB 601, which set a World Speed Record of 469.2mph (750.7km) in a Messerschmitt Bf 109R in 1939, was specially boosted to develop a maximum of 1,800hp at 3,500rpm as opposed to the 1,050hp at 2,400rpm for the then standard engine. The DB 603 had commenced production in May 1942. It powered a number of aircraft including the twin-engined Dornier Do 217 fighter/bomber N and M versions, the Heinkel He 219 Uhu night-fighter, Messerschmitt Me 410 heavy-fighter, the Do 335 tandem-engined heavy-fighter, and the single-engined Focke-Wulf Ta 152C high-altitude fighter interceptor. (The Ta 152C only entered service in January 1945.) By the latter stages of the war the development of the DB 603 had, pretty much, been pressed to the limit, resulting in a take-off rating of 2,400bhp. A later version, designated as the DB 603N, was upgraded with a new cylinder head and a two-stage two-speed blower, although without intercooler, giving 2,800 to 2,900hp. But it never entered production.
Diagram of the powerful BMW 801A air-cooled radial.
It is interesting to note that the quest to reduce drag, resulting in the cramping of the engine, had caused considerable headaches for the engineers at Daimler-Benz, in particular when it came to its maintenance or installation. Unlike BMW they had never been given the opportunity to conduct full-scale testing in the LFA Völkenrode wind tunnels. Despite this, an experimental installation of the DB 603E in a Focke-Wulf Fw 190 had, it was claimed, resulted in a speed of 420mph (676km/h) at 18,000ft (5,500m). Although this was not that much faster than the standard Fw 190 A-8 fitted with the BMW 801 D-1 at 408mph (656km/h), it was only a whisker slower than the 426mph (685km/h) claimed for the Jumo 213 A-1 engined Fw 190 D-9.
Fedden was informed that all work on the DB 604, a twenty-four-cylinder 43l X-configured engine, had been stopped in 1940 by the Air Ministry which did not consider the design to be sufficiently promising to merit further resources. He also learned that some studies were undertaken on an even bigger thirty-six-cylinder engine as well.
During the war years the production of Daimler-Benz engines had peaked in the summer of 1944 when around 3,000 were produced by 50,000 people working at the parent and subsidiary firms.
THE SWING-PISTON ENGINE
An alternative approach to the piston engine was the Lutz toroidal or swing-piston engine which the mission had come across in the engine department at Völkenrode, albeit in pieces. The concept of the toroidal is not dissimilar to Felix Wankel’s rotary engine which it pre-dates. Wankel’s design featured a three-sided rotor which moved about an eccentric shaft within an oval-shaped chamber to create a cycle of three combustion chambers, directing the force of the gas pressure to drive the shaft directly. In the swing-piston engine the pistons move in a circular motion inside a ring-shaped cylinder, moving towards or away from each other to provide compression and expansion. During the war Otto Lutz had developed an experimental six-cylinder toroidal in association with the Bussing Company of Braunschweig. The aim was to create an operational system consisting of compact multi-sectioned units, but Fedden was characteristically scathing regarding its potential as an aero engine:
It is understood that experimental work was being done by Junkers and also by Bosch and Mahle. Opinions varied considerably as to the promise this work held out, but the general feeling was that it introduced rather more new problems than it solved old ones, and that its chances of success in the aircraft field were not particularly rosy.
The concept has recently enjoyed something of a revival and Raphial Morgado’s Massive Yet Tiny (MYT) Toroidal engine won first prize in the 2005 Emhart-Nasa Tech Briefs Design Contest.
Fedden felt that at the time of Germany’s collapse the best that the aero engine industry could offer were the twelve-cylinder liquid-cooled and the fourteen-cylinder air-cooled series in the power range of 2,000 to 2,4000hp:
They had a number of other developments, but, as happened in other countries, they had found high power piston engine development very slow and extremely difficult. They were also hampered by a number of bad technical decisions from the RLM, and a policy of stopping and starting certain production types. Furthermore, their development was being harassed all the time in continually increasing measure by our bombing. Under wartime conditions they could not see that a 5,000hp engine could be got into series production in less than four years, and this would probably have been the Jumo 222 or the BMW 803 in their opinion, although the latter appeared to the Mission to be a poor effort which was unlikely ever to become a classic type.
In 1944 the RLM ordered that all development work on piston engines should be halted in order to concentrate efforts and resources on the turbojets. Spurred on by an increasing atmosphere of panic, limited piston engine development recommenced in January 1945, but the drive seems to have been spasmodic.
Summing up his impression of German piston engine development Fedden concluded by saying:
It is not felt that there is much to learn from Germany about piston engine development, of a general policy nature, except the emphasis placed on the results of full scale wind tunnel tests, for both air-cooled and liquid-cooled types, the necessity for foolproof single-lever control, and the feeling that fuel injection had not only been worth while, but becomes progressively more important as the number of cylinders increases. All this lines up well with our own reactions to the problem of the higher power piston engine.
Preserved BMW 801 D2. (Rottweiler)
ENGINE IGNITION, FUEL INJECTION AND VARIABLE PITCH PROPELLERS
In addition to the engines themselves, the Fedden Mission also examined German development work on engine ignition, fuel injection and variable pitch propellers. When it came to fuel injection the mission had little to report as they had been unable to get hold of the right people at Junkers, or at Deckel in Munich, although in Stuttgart they had more luck and were able to talk to Dr Heinrich, a designer from the Bosch injection development department at Reutlingen. In general Heinrich said that, apart from producing larger capacity units, their efforts to make improvements to the fuel injection pumps had not borne fruit. The closed Bosch nozzle, as used on the BMW 801, had performed well enough, he admitted, although it was complicated to manufacture. At Dessau, Junkers had tried various types of nozzles but stuck to the open type fitted on the Jumo 213.
On the subject of sparking plugs Fedden’s experts fared better, with information obtained from BMW, Daimler-Benz and Bosch, although not from the other two major plug manufacturers, Beru and Siemens. Apart from special projects the main development work had been on improved operation at higher altitudes or under hotter engine conditions, either through better electrical insulation or more efficient cooling.
All work on propellers was halted once the turbojets had been allocated the highest priority, but it was picked up later on when the development of piston engines resumed. The only propeller activity investigated by Fedden’s team was that of the Vereinigte Deutsche Metallwerke (VDM) company. At Göttingen they had been able to talk to Dr J. Stüper who had done a considerable amount of test flying as part of the development of VDM’s braking propeller. This consisted of three blades connected by links through the hub to the inner member of a large bearing, which slid on an extension at the rear of the hub. An outer bearing member is fitted in a hinged yoke which is tilted by threaded rods controlled by two electric motors to alter the pitch of the propellers. The rate of pitch change was 2° per second for constant speeding and 60° to 100° per second for reversing. Its main purpose was as a brake in the windmill brake position. According to Stüper the system had been very satisfactory and production was due to start in early 1945 for the Dornier Do 335, Do 317 and the Fw 190.
The mission also spoke to Dr Eckert, technical director of Continental Metall Gesellschaft (CMG), an off-shoot of ?VDM which had taken over the propeller work, although forgings were still being supplied by VDM. He confirmed Stüper’s observations on reversing propellers and added that they were considering replacing the electric motors with hydraulic cylinders to enable counterweights to be dispensed with.
CMG had another type of reversing propeller under construction with the gearing rearranged in such a way that, with a four-bladed prop, it would be possible to put the propeller into braking pitch by decreasing pitch on two blades while simultaneously increasing it on the other two blades. The main advantage of this would be the elimination of the over-speeding of engines during braking.
CMG had also commenced research into hollow steel blades. On the first trials the welding on the leading and trailing edges failed. They then tried a blade made up of a hollow rectangular tube with the edges welded on. Like many other firms CMG had suffered badly as a result of the Allied bombing campaign and had been forced to move its machine shops several times. Fedden inspected one which had been relocated to a railway tunnel at Kasselborn, about 22 miles (35km) from Frankfurt, but found little of interest.
The forging of the metal propeller blades was done by VDM at their works in Heddernheim on the north-west edge of Frankfurt. The process of making a propeller began with a 31in (80cm) cylinder of metal alloy, about 13in (33cm) thick, which was heated to 750°F (400°C) and extruded into oval sections 5.9in (15cm) on its widest axis and 19ft 8in (6m) long. These lengths were cut into six, heated again to 750°F (400°C) and passed through rollers to produce tapered forging blanks. These were heated, to 840°F (450°C) this time, and formed into shape by a massive 15,000-ton press. The largest propeller made by VDM was a four-blader of 14ft 9in (4.5m) diameter for the DB 613 engine. Towards the end of the war a shortage of aluminium necessitated the increased use of wooden blades.
As previously mentioned, BMW’s Bruckmann had also advocated the development of turbine engines to drive propellers, which he thought would eventually come into common use for long-range aircraft.
Focke-Wulf Fw 190A. The A series was fitted with variants of the BMW 801 radial. (USAF)
Messerschmitt Bf 110 with a pair of DB 601 liquid-cooled inverted V12s.
THE DORNIER Do 335 PFEIL
Of all of the Luftwaffe’s piston-engined propeller-driven aircraft the most unusual, but also the fastest – and potentially the fastest piston-engined fighter ever – was undoubtedly the Dornier Do 335. Known as the Pfeil (‘arrow’), this featured a distinctive tandem push-pull configuration with engines and propellers at both the front and back. Its origins can be traced back to Claude Dornier’s earlier experiments with tandem engines on most of his multi-engined flying boats, including the mighty Do.X which had been fitted with Fedden’s Jupiters at one stage.
On the Do 335 one DB 603 engine was mounted in the nose in the conventional manner, with a second one within the fuselage behind the cockpit driving a propeller in the tail via an extension shaft. Dornier had patented this configuration in 1937 and a concept demonstrator was built by Schemp-Hirth, the Göppingen Gö 9, which flew successfully in 1940. The RLM, however, showed little enthusiasm for a tandem fighter and required Dornier to concentrate its efforts on its range of bombers. Then in May 1942 Dornier’s updated version of the push-pull configuration, the P.231, beat off the competition in meeting the ministry’s requirement for a single-seat high-speed bomber/intruder. The P.231 became the Do 335 but by the autumn of 1942 the RLM had decided that the bomber/intruder was no longer required and instead a multi-role fighter based on the same general layout should go ahead. For a fast fighter aircraft there were distinct advantages in the tandem design, in particular the power of a twin-engined aircraft without the greater frontal area and associated drag of mounting an engine on each wing. It also placed the weight of the engines on or near the aircraft’s centreline, increasing the roll rate and eliminating the asymmetrical thrust normally caused by a single-engine failure.
The push-pull configuration of the Dornier Do 335 looked all wrong, but the reduced drag resulted in an extremely fast fighter aircraft. Only a handful were built. (NARA)
In terms of appearance the Do 335 looked like no other aircraft. For a start it was very big for a fighter; 45ft 5in (13.85m) long and, riding on a tall tricycle undercarriage, it was high at 16ft 4in (5m) to the top of the cruciform tail fin. This tail extended below the fuselage to protect the rear propeller from possible ground strikes. In the event of a pilot bail-out, explosive bolts would jettison the upper tail and rear propeller, although at least one aircraft was fitted with an ejector seat.
Flight testing commenced on 26 October 1943 when the Do 335 V1 prototype took off from Dornier’s Oberpfaffenhofen airfield. Despite the loss of the second prototype due to a fire in the rear engine, the aircraft demonstrated good handling qualities with an exceptional rate of climb, reaching 26,250ft (8,000m) in under fifteen minutes, and a blistering maximum speed of 474mph (763km/h) at 21,300ft (6,500m). Production of the Do 335 A-1 began in late 1944, and several variants were planned including two-seater night-fighter and trainer versions. The Do 335 B-1 saw an upgrade in weaponry from the 15mm MG 151/15 cannon to the 20mm cannon, and the Do 335 B-2 was to have an additional pair of 30mm MK 103s installed in the wings. The Do 335 B-3 was basically a Do 335 B-1 fitted with the more powerful 2,100hp DB 603LA engine.
By the time the US Army overran the Oberpfaffenhofen factory in April 1945 just a handful of the Do 335 fighter-bombers and two A-12 conversion trainers had been completed. Including the various prototype aircraft a total of only thirty-seven Do 335s were ever built and the sole surviving example, a Do 335 A-0 pre-production model, is displayed at the Smithsonian National Air and Space Museum in Washington D.C.