The Army Air Service was created in 1907. It was an element of the Signal Corps until about 1927, when it became the Army Air Corps. In 1942 it was redesignated the Army Air Forces (AAF) but the new designation never quite caught on and it was popularly known, and referred to, as the Air Corps until it became the U. S. Air Force - a separate branch of service - in 1947.
Organization has been essentially the same since World War I. The smallest unit is a flight, which has traditionally consisted of from 4 to 6 aircraft and may be commanded by a lieutenant or captain. A squadron is made up of two or more flights and is usually commanded by a major or lieutenant colonel. A group consists of two or more squadrons commanded by a lieutenant colonel or colonel (groups have been largely replaced by wings since WWII). A wing may consist of several groups and/or squadrons, commanded by a colonel or brigadier general. An air division may consist of any combination of wings and/or groups and squadrons commanded by a major general. An air force may consist of any combination of all the above subordinate units, usually commanded by a major general or lieutenant general. A command may consist of any combination of the above subordinate units, commanded by a lieutenant general or general.