June 20, 1941. The Army Air Corps becomes the Army Air Forces.

On this day in 1941, expressing its growing significance and the need to increase its autonomy, the United States Army Air Corps became the United States Army Air Forces.

The date I learned here:

Today in World War II History—June 20, 1941

The evolution had been occurring for some time so the increased degree of separation from the rest of the Army was not surprising.  None the less, it stopped short of full separation, as Air Force would not become a separate branch of service until 1947.