The most unusual conversion of the B-17G was the JB-17G converted to an engine test bed. The nose section was removed and replaced with a strengthened mount for a fifth engine.
Generally considered the definitive B-17 design, all changes made in the B-17F production run were incorporated into the final version. These included a Bendix chin turret, an innovation derived from the unsuccessful YB-40 escort version, bringing defensive armament to 13 .50 caliber (half-inch or 12.7 mm) machine guns, and a revised tail gun position (referred to as the "Cheyenne" configuration after the modification center where it was introduced) in which the guns were mounted in a remotely-operated turret. Some 8,680 were built, and dozens were converted for several different uses:
- CB-17G. Troop transport version, capable of carrying 64 troops.
- DB-17G. Drone variant.
- JB-17G. Engine test-bed.
- QB-17L. Target drone.
- QB-17N. Target drone.
- RB-17G. Reconnaissance variant.
- SB-17G. Rescue version, originally designated B-17H. Featured A-1 lifeboat under fuselage. After World War II, armament on the SB-17Gs was removed; it was reinstated when the Korean conflict began.
- TB-17G. Special duty training version.
- VB-17G. VIP transport.
- PB-1. This designation was given to one B-17F and one B-17G. They were used by the U.S. Navy for various test projects.
- PB-1G. This designation was given to 17 B-17Gs used by U.S. Coast Guard as air-sea rescue aircraft.
- PB-1W. This designation was given to 31 B-17Gs used by the U.S. Navy as the first airborne early warning aircraft/ AWACS.
Eighty-five B-17Gs were transferred to the Royal Air Force, where they were used as the Fortress III. They operated with two squadrons of Bomber Command's No. 100 Group RAF at RAF Sculthorpe, where they were used for electronic countermeasures missions to confuse and jam enemy radar. They were also used as decoys during night bombing attacks. They took part in various such operations until the units were disbanded in July 1945.