United States Air Force
The U.S. War Department created the first antecedent of the U.S. Air Force, as a part of the U.S. Army, on 1 August 1907, which through a succession of changes of organization, titles, and missions advanced toward eventual independence 40 years later. In World War II, almost 68,000 U.S. airmen died helping to win the war, with only the infantry suffering more casualties. In practice, the U.S. Army Air Forces (USAAF) was virtually independent of the Army during World War II, and in virtually all ways functioned as an independent service branch, but airmen still pressed for formal independence. The National Security Act of 1947 was signed on 26 July 1947 by President Harry S Truman, which established the Department of the Air Force, but it was not until 18 September 1947, when the first secretary of the Air Force, W. Stuart Symington, was sworn into office that the Air Force was officially formed as an independent service branch.
Air National Guard
Being "local" ground forces affiliated with the Army, militias were considered state-centric/territorial-centric in nature, this versus naval forces, which were considered wholly activities of the federal government. This distinction accounts for why there are no National Guard components in the U.S. Navy, U.S. Marine Corps or U.S. Coast Guard. Because the present day U.S. Air Force evolved from the U.S. Army, it was only natural that a separate Air National Guard would be established with the divestiture of the former U.S. Army Air Forces and its establishment as a separate and independent U.S. Air Force in 1947.