<P> The United States Air Force became a separate military service on 18 September 1947 with the implementation of the National Security Act of 1947 . The Act created the National Military Establishment, later renamed the United States Department of Defense, which was composed of four of the five branches, the Army, Marine Corps, Navy, and a newly created Air Force . Prior to 1947, the responsibility for military aviation was divided between the Army for land - based operations and the Navy and Marine Corps for sea - based operations from aircraft carrier and amphibious aircraft . The Army created the first antecedent of the Air Force on 1 August 1907, which through a succession of changes of organization, titles, and missions advanced toward eventual separation 40 years later . The predecessor organizations leading up to today's U.S. Air Force are: </P> <Ul> <Li> Aeronautical Division, Signal Corps (1 August 1907--18 July 1914) </Li> <Li> Aviation Section, Signal Corps (18 July 1914--20 May 1918 </Li> <Li> Division of Military Aeronautics (20 May 1918--24 May 1918) </Li> <Li> Air Service, U.S. Army (24 May 1918--2 July 1926) </Li> <Li> U.S. Army Air Corps (2 July 1926--20 June 1941) * </Li> <Li> U.S. Army Air Forces (20 June 1941--17 September 1947) * * </Li> </Ul>

When did the air force leave the army