The Boeing B-17 Flying Fortress is a four-engined heavy bomber developed in the 1930s for the United States Army Air Corps (USAAC). Competing against Douglas and Martin for a contract to build 200 bombers, the Boeing entry (prototype Model 299/XB-17) outperformed both competitors and exceeded the air corps' performance specifications. Although Boeing lost the contract (to the Douglas B-18 Bolo) because the prototype crashed, the air corps ordered 13 more B-17s for further evaluation. From its introduction in 1938, the B-17 Flying Fortress evolved through numerous design advances,[4][5] becoming the third-most produced bomber of all time, behind the four-engined Consolidated B-24 Liberator and the multirole, twin-engined Junkers Ju 88.

Credits:
Mr Shenanigans for autoleading turrets
SnoWflakE0S for Bomsight

AG:1 For bomb bays
AG:6 For dropping ball turret