This is the XB-39, a one off prototype that modified one of the first YB-29s to use the new Allison twin V12 V3420 engines to replace the troublesome Wright R-3550s in November 1943. \
This was a modification done to other craft, was done to the Douglas XB-19 earlier in the year
These Allisons didn't have the cooling problems of the Wright, and were relatively trouble free during its role as cargo transport for the rest of the War
Re-engining theXB-39 was a low priority project at GM, who was more interested in using those engines in the P-75, and GE wouldn't release turbochargers for use on the XB-39 till late 1944, and was flown at first without them, after the P-75 was cancelled
With the new engines, it was around 40 knots faster.
Could this been done faster at Seattle, than by GM, who had little airframe experience?
yeah.