Boeing JB-17G test bed for turbo prop engine, in flight near Edwards Air Force Base, Calif., on Aug. 9, 1956.
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.