The 519th Fighter-Interceptor Squadron is an inactive United States Air Force unit. Its last assignment was with the 521st Air Defense Group at Sioux City Municipal Airport, Iowa, where it was inactivated on 18 August 1955. The squadron was first activated during World War II as the 637th Bombardment Squadron, a dive bomber unit, but was disbanded before participating in combat in a general reorganization of Army Air Forces training units. It was reconstituted during the Cold War as a fighter interceptor unit supporting the air defense of the United States.