<Li> Lt. Gen. Delos C. Emmons, 20 June 1941--17 December 1941 </Li> <Li> Maj. Gen. Carl A. Spaatz--c . January 1942--5 May 1942 </Li> <Ul> <Li> Aeronautical Division, Signal Corps 1 August 1907--18 July 1914 </Li> <Li> Aviation Section, Signal Corps 18 July 1914--20 May 1918 </Li> <Li> Division of Military Aeronautics 20 May 1918--24 May 1918 </Li> <Li> Air Service, U.S. Army 24 May 1918--2 July 1926 </Li> <Li> U.S. Army Air Corps 2 July 1926--20 June 1941 * </Li> <Li> U.S. Army Air Forces 20 June 1941--18 September 1947 * </Li> <Li> United States Air Force 18 September 1947--present </Li> </Ul> <Li> Aeronautical Division, Signal Corps 1 August 1907--18 July 1914 </Li>

When did the us army air corps become the us air force