Sept 18, 1947. Although its heritage dates back to 1907 when the army first established military aviation, the US Air Force became a separate military service on this date. Responsible for providing an air force that is capable, in conjunction with the other armed forces, of preserving the peace and security of the US, the department is separately organized under the secretary of the air force and operates under the authority, direction and control of the secretary of defense. Read more at http://www.airforce.com/learn-about/history/.
Image: Public Domain, USAF via Wikipedia.org