Hap Arnold was a five-star General of the United States Army during World War II. He is the only person to ever hold a five-star rank in two branches of the U.S. Armed Forces, having held both the Army and Air Force ranks. Arnold was an aviation pioneer, being the first military aviator to fly a plane in combat and the first to command an air force. He was also the first to receive the Medal of Honor for his service in World War I. Arnold's nickname, 'HAP', was derived from his initials, H.A.P.