The Boeing B-17 Flying Fortress is an American four-engined heavy bomber aircraft developed in the 1930s for the United States Army Air Corps (USAAC). A fast and high-flying 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 American four-engined Consolidated B-24 Liberator and the German multirole, twin-engined Junkers Ju 88. It was also employed as a transport, antisubmarine aircraft, drone controller, and search-and-rescue aircraft.

Heavy bombers are bomber aircraft capable of delivering the largest payload of air-to-ground weaponry and longest range of their era. Archetypal heavy bombers have therefore usually been among the largest and most powerful military aircraft at any point in time. In the second half of the 20th century, heavy bombers were largely superseded by strategic bombers, which were often even larger in size, had much longer ranges and were capable of delivering nuclear bombs.