the United States Army Air Service became the United States Army Air Corps on 2 Jul 1926.
On 1 March 1935, still struggling with the issue of a separate air arm, the Army activated the General Headquarters Air Force for centralized control of aviation combat units within the continental United States, separate from but coordinate with the Air Corps. The separation of the Air Corps from control of its combat units caused problems of unity of command that became more acute as the Air Corps enlarged in preparation for World War II. This was resolved by the creation of the Army Air Forces (AAF), making both organizations subordinate to the new higher echelon.
The United States Army Air Forces was created in June 1941 from both the Air Corps and General Headquarters Air Force.
The Air Corps ceased to have an administrative structure after 9 March 1942, but as "the permanent statutory organization of the air arm, and the principal component of the Army Air Forces," the overwhelming majority of personnel assigned to the AAF were members of the Air Corps.
To avoid the headache of calling some people Air Corps, other Army Air Forces, Units Army Air Forces, etc, we decided to go with calling them all Army Air Forces after June 1941 until they became the Air Force on 18 Sep 1947. note in the above paragraph that it states the Air Corps is the principal component of the Army Air Forces, so it makes sense to call them by the parent, not the sub ordinate unit.....