Starting in 1943 and continuing after the end of World War II, some B-17Gs were converted for second line duties. The majority of the modified aircraft were made into transport variants. The CB-17G was initially designed as a troop transport capable of carrying up to 64 troops. The VB-17G was a VIP transport for high level staff officers.
The SB-17G, initially designated B-17H, was modified for use as a rescue aircraft. The RB-17G, initially designated F-9C, was a reconnaissance variant. A few aircraft were converted for use as drone directors and designated DB-17G.
The most unusual conversions were three B-17Gs converted to engine test beds. The nose section was removed and replaced with a strengthened mount for a fifth engine. The Pratt & Whitney XT-34, Wright XT-35, Wright R-3350 and Allison T-56 engines were all flight tested on JB-17Gs.