WaltBJ wrote:

Along the same lines I have seen a picture of the XB39, a B29 with the
big 'double Allison' 24 cylinder engine. (2 V12s side by side geared
to a common drive shaft.)
I presume engine bugs forestalled its further development.
Walt BJ


From the USAF Museum web site.
"The XB-39 project was basically a proof-of-concept project to
demonstrate performance with liquid cooled 'Vee' engines. It was also
insurance against shortages of the production engine. Only XB-39 was
built; it was delivered the the US Army Air Force in early 1944 for
testing. Most of the problems with the standard B-29 production version
were fixed by mid-1944 and no orders for the XB-39 were placed."
http://www.wpafb.af.mil/museum/resea...bers/b3-88.htm