He married Leticia Christina Cox on 5 February 1942 in Texas. They had one son, William Douglas Barnum, who was born on 28 July 1946 in Denton, Texas. Billie enlisted in the Army Air Corps on 26 September 1940 and reached the rank of Major before the end of World War II. After the war, he moved his family to the Virginia suburbs of Washington, DC. There, he worked first at the Pentagon, as a civilian employee of the Comptroller's Department of the Air Force, and later as a NASA employee. He also remained a member of the Air Force Reserve, and had earned the rank of Lieutenant Colonel by the time of his death.
in Arlington National Cemetery.