The Enlightenment wrote in message
om...
I was going to start a new thread on this but there is a case for
putting duel stage supercharged packard merlins on a least some B17s
and B24s simply to save refractory alloys. I don't think altitude
performance would have suffered and speed might have increased. The
Allison B17 was faster than the radial B17 by about 25mph after all.

Fahey listed the XB-39-GM (YB-29 conversion to V-3420) as having
a speed in excess of 400mph (~405 with a 282 cruise per USAFM).
This compares with the YB-29's original 368 top speed & 247 cruise.
Range got a slight bump with the Vs of 6290 miles vs 5850 miles.
(Note: the production B-29 was slower with 357mph & 220 and a
max ferry of 5800.)

The XB-38 (B-17E with V-1710 engines) was about 327 mph top-end
and cruising at 226 mph. Compare to the original's 317 & 226. The
XB-38 did carry a bit more fuel and max ferry range went from 3200
to 3600 miles.