The B-17's turbocharged Wright R-1820 Cyclone 9 engines were upgraded to increasingly more powerful versions of the same powerplants multiple times throughout its production, and similarly, the number of machine gun emplacement locations was increased to enhance the aircraft's combat effectiveness.
B-17s turboladede Wright R-1820 Cyclone 9 motorer blev flere gange igennem produktionsforløbet opgraderet til stadigt kraftigere versioner af samme motor, og på tilsvarende vis blev antallet af maskingeværstillinger forøget, for at forbedre flyets effektivitet i kamp.