A modified B-17 is used to test the Pratt & Whitney XT-34 turboprop engine.  This engine would go on to power the Super Guppy, the Boeing Stratotanker, the Lockheed Constellation, the Douglas Globemaster and the Douglas Cargomaster. With water injection, the final variant, the T34-P-9W, was rated up to 7500 hp.  The B-17 in this photo was rebuilt to stock configuration as "Liberty Belle", but was lost in a fire after a forced landing on 2011.