What is now the United States Air Force (USAF) was formerly a part of the US Army, namely the United States Army Air Corps or USAAC. It was established under this name by an act of Congress on July 2, 1926. During World War II its role grew greatly; the Air Corps eventually became subordinate to the United States Army Air Force (USAAF) when the latter was established on June 20, 1941. The required Congressional disestablishment of the Air Corps itself did not occur until 1947.
See also: Boeing P-26