The United States Army Air Forces (USAAF or AAF) was the military aviation arm of the United States of America during and immediately after World War II, and the direct predecessor of the United States Air Force. The AAF was a component of the United States Army, which in 1942 was divided functionally by executive order into three autonomous forces: the Army Ground Forces, the Services of Supply (which in 1943 became the Army Service Forces), and the AAF. Each of these forces had a commanding general who reported directly to the Chief of Staff of the United States Army.
The AAF administered all parts of military aviation formerly distributed among the Army Air Corps, General Headquarters Air Force, and ground forces corps area commanders, and thus became the first air organization of the U.S. Army to control its own installations and support personnel. In practice, the AAF was virtually autonomous inside the Army. The peak size of the AAF was over 2.4 million men and women in service and nearly 80,000 aircraft in 1944, and 783 domestic bases in December 1943. By VE Day it had 1.25 million men stationed overseas and operated from more than 1,600 airfields worldwide.
The Air Corps became the Army Air Forces in June 1941 to provide the air arm a greater autonomy in which to expand more efficiently, and to provide a structure for the additional command echelons required by a vastly increased force. Although other nations already had separate air forces independent of the army or navy (such as the British Royal Air Force and the German Luftwaffe), the AAF remained a part of the United States Army until the United States Air Force came into being in September 1947.
However, in its expansion and conduct of the war, the AAF became more than just an arm of the greater organization. By the end of World War II the AAF had become virtually an independent service. By regulation and executive order, the AAF was a subordinate agency of the War Department tasked only with organizing, training, and equipping combat units, limited in responsibility to the continental United States, as were the Army Ground Forces and the Army Service Forces. In reality, Headquarters AAF controlled the conduct of all aspects of the air war in every part of the world, determining air policy and issuing orders without transmitting them through the Chief of Staff. This "contrast between theory and fact is...fundamental to an understanding of the AAF."
Read more about USAAF: Expansion, See Also, Lineage of The United States Air Force, Notes