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 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.