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.
On July 26, 1947 President Harry S. Truman signed the National Security Act, which created a Department of the Air Force equal to the Department of the army and the Department of the Navy; a National Military Establishment under the secretary of defense; and an Air National Guard as a reserve component of the Air Force.
Wikipedia Multimedia Encyclopedia and Air Force Historical Research Agency “One Hundred Years”