BU-U, 214 Sq., 100 Group, RAF. The Boeing B-17 Flying Fortress is a four-engine heavy bomber aircraft developed for the United States Army Air Corps (USAAC). The B-17 Flying Fortress went on to enter full-scale production and was considered the first truly mass-produced large aircraft, eventually evolving through numerous design advancements. The B-17 was primarily employed by the United States Army Air Force (USAAF) in the daylight precision strategic bombing campaign of World War II against German industrial, civilian and military targets.