The U.S. War Department created the first antecedent of the U.S. Air Force in 1907, which through a succession of changes of organization, titles, and missions advanced toward eventual separation 40 years later. In World War II, almost 68,000 U.S airmen died helping to win the war; only the infantry suffered more enlisted casualties. In practice, the U.S. Army Air Forces (USAAF) was virtually independent of the Army during World War II, but officials wanted formal independence. The National Security Act of 1947 was signed on on 26 July 1947 by President Harry S Truman, which established the Department of the Air Force, but it was not 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.
What does the USAAF stand for? 
U.S. Army Air Forces