18 September 1947 – The United States Army Air Forces separates from the United States Army and becomes an independent armed service, the United States Air Force. 
Initially formed as a part of the United States Army on 1 August 1907, the USAF was established as a separate branch of the U.S. Armed Forces on 18 September 1947 with the passing of the National Security Act of 1947. It is the second youngest branch of the U.S. Armed Forces[a] and the fourth in order of precedence. The U.S. Air Force articulates its core missions as air superiority, global integrated intelligence, surveillance and reconnaissance, rapid global mobility, global strike, and command and control. (1)
(1) Wikipedia