That is one of the two Boeing 299Zs. These were B-17Gs modified to be
engine testbeds for Curtiss-Wright and Pratt & Whitney. They had the
cockpit moved back and mounting provisions for a fifth engine in the
nose. Both were used to test early turboprops. CW's ended up as a
firebomber and was destroyed in a crash. P&W's ended up in the New
England Air Musuem.