When Germany invaded Poland in September 1939, the Air Corps had 26,000 officers and airmen and a heavy bomber force of only 23 B-17s. Chief of Air Corps Arnold had used President Roosevelt's support and British and French orders for 10,000 additional aircraft to launch a huge expansion of the aviation industry. With the fall of France in June 1940, Roosevelt ordered an Air Corps of 50,000 aircraft and 54 combat groups. Congress appropriated $2 billion, eventually, to insure funding for both strategic and tactical air forces. In March 1941 the Air Corps expanded to 84 groups. These actions and events presaged what would become the largest air force in the world equipped with the most modern aircraft available. By December 1941, however, the Army's air force still had only 3,304 combat aircraft, but World War II mainstays such as P-51 Mustang and P-47 Thunderbolt fighters and the B-29 Superfortress bomber still were not operational. All would become part of the U.S. Army Air Forces (USAAF) led by Major General Hap Arnold, established under Army Regulation 95-5 on June 20, 1941, with the Air Corps and the Air Force Combat Command (formerly the GHQAF) as subordinate arms. Less than a year later, Army Chief of Staff George Marshall made the USAAF coequal to the Ground Forces and Services of Supply.
In August 1941, at the behest of the War Department, USAAF Chief Arnold directed four former faculty members of the Air Corps Tactical School to devise an air plan against America's potential adversaries. Lieutenant Colonels Kenneth Walker and Harold George and Majors Haywood Hansell and Laurence Kuter of the newly-formed Air War Plans Division (AWPD) identified in their plan 154 "chokepoint" targets in the German industrial fabric, the destruction of which, they held, would render Germany "incapable of continuing to fight a war." A lack of intelligence prevented the design of a similar plan against Japan. The four planners calculated that the desired air campaign would require 98 bomber groups―a force of over 6,800 aircraft. From their recommendation General Arnold determined the number of supporting units, air-craft, pilots, mechanics, and all other skills and equipment the USAAF would need to fight what became World War II. The 239 groups estimated came close to the 243 combat groups representing 80,000 aircraft and 2.4 million personnel that actually formed the USAAF in 1944 at its wartime peak. The planners had also assumed that they would not have to initiate their air plan, known as AWPD/1, with a complete 98-group force until April 1944. However, they were not allowed the luxury of time. When the Japanese attacked Pearl Harbor four months after the air plan's submission to the War Department, an ill-equipped USAAF found itself thrust into the greatest war in human history.