The Boeing B-17 Flying Fortress is a four-engined heavy bomber developed in the 1930s for the United States Army Air Corps (USAAC). Relatively fast and high-flying for a bomber of its era, the B-17 was used primarily in the European Theater of Operations and dropped more bombs than any other aircraft during World War II. It is the third-most produced bomber of all time, behind the four-engined Consolidated B-24 Liberator and the multirole, twin-engined Junkers Ju 88. It was also employed as a transport, antisubmarine aircraft, drone controller, and search-and-rescue aircraft.