Arnold was the US general who commanded the USAAF in all theaters throughout the war. In 1936 he was appointed Assistant Chief of the Air Corps and became Chief of Air Staff in 1938. Although no funds were available he persuaded the US aviation industry to step up the production of airplanes and to prepare plant and training facilities in anticipation of the rush for new aircraft thereby insuring production capacity. Aircraft production in the USA grew from less than 6000 per annum to 262,000 during the years 1940-44. World War II saw a massive growth in the USAAF and Arnold was behind every change. He supervised training programs for pilots and by the end of the war, Air Force personnel had grown from 21,000 in 1935 to over two million in 1944.
He served on the US Joint Chiefs of Staff Committee and Combined Chiefs of Staff Committee of the Allies. He recommended strong support for the British and was opposed to the policy of American isolationism. He considered air power would prove the decisive factor in any future conflict. Arnold firmly believed in the war-winning capability of strategic bombing of specific targets and this brought him into conflict with his British counterparts who favored area bombing. This problem was resolved at the Casablanca Conference in January 1943 when it was agreed that both methods would be adopted.
He was a popular figure whose nickname ‘Hap’ or ‘Happy’ was well- deserved. He managed to develop a working relationship with the solitary Chief of Air Staff, PORTAL, which was essential to Allied success. In 1944 he was made a full General of the Army and when the USAAF was made into a separate force equal to the Army and Navy, he was its first Five-Star General.