The U.S. Air Force was officially founded September 18, 1947, and in the decades since it has established itself as an air force second-to-none. Yet the Air Force's history and heritage goes back the turn of the twentieth century.
Between 1909 -- when the U.S. military purchased its first aircraft -- and 1947, the U.S. Air Force did not exist as a separate and independent military service organization. It went through a series of designations: Aeronautical Section, Signal Corps (1909); Aviation Section, Signal Corps (1914); United States Army Air Service (1918); United States Army Air Corps (1926), and United States Army Air Forces (1941).
WWII illustrated the value of airpower, and the need to change the basic organization of the U.S. Military Forces. The result was the creation of a single Department of Defense with a strong Joint Chiefs of Staff with Army, Navy, and Air Force chiefs.
In 1947, President Truman signed the National Security Act which established this new defense organization, and along with it the creation of the U.S. Air Force as an independent service, equal to the U.S. Army and the U.S. Navy. (Information provided by www.military.com)