The 521st Air Defense Group is a disbanded United States Air Force organization. Its last assignment was with the 31st Air Division at Sioux City Municipal Airport, Iowa, where it was inactivated in 1955. The group was originally activated as a support unit for the 310th Bombardment Group at the end of World War II in Italy and then redeployed to the Unitded States where it was inactivated in 1945. The group was activated once again in 1953, when ADC established it as the headquarters for a dispersed fighter-interceptor squadron and the medical, maintenance, and administrative squadrons supporting it. It was replaced in 1955 when ADC transferred its mission, equipment, and personnel to the 53d Fighter Group in a project that replaced air defense groups commanding fighter squadrons with fig