One of the rare B-17 conversions, a USAF test bed for turbo prop engines. Three B-17Gs were 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.
Allison's T56 series(then T-38) was first flown in the nose of a B-17 test-bed aircraft in 1954. It was originally developed by the Allison Engine Company for the Lockheed C-130 transport entering production in 1954 as the T-56. It is now produced under Rolls-Royce which acquired Allison in 1995.
The T56 was also installed on the P-3 and E-2/C-2 aircraft, as well as civilian airliners such as the Lockheed Electra and Convair 580. A shipboard version, the 501K engine, is used to generate electrical power for some destroyer-class ships in the U.S. Navy.The commercial version is designated 501-D. With an unusually long and numerous production run, over 18,000 engines have been produced since 1954. It has logged over 200 million flying hours.
Canadian Aviation Blog
(U.S. Air Force photo)