The United States Air Force became a separate military service on September 18, 1947, with the implementation of the National Security Act of 1947. The Act created the United States Department of Defense, which was composed of three branches, the Army, 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, for sea-based operations from aircraft carrier and amphibious aircraft. The Army created the first antecedent of the Air Force in 1907, which through a succession of changes of organization, titles, and missions advanced toward eventual separation 40 years later. The predecessor organizations of today's U.S. Air Force are:
- Aeronautical Division, U.S. Signal Corps (August 1, 1907 to July 18, 1914)
- Aviation Section, U.S. Signal Corps (July 18, 1914 to May 20, 1918)
- Division of Military Aeronautics (May 20, 1918 to May 24, 1918)
- U.S. Army Air Service (May 24, 1918 to July 2, 1926)
- U.S. Army Air Corps (July 2, 1926 to June 20, 1941) and
- U.S. Army Air Forces (June 20, 1941 to September 17, 1947)