Stephen Harding wrote:

: Anyone know why the modification didn't go anywhere?

The XB-38 was lost before much testing could be
done. It was slightly faster, but not enough
to make it worth the effort. And the V-1710
was of course much more vulnerable to combat
damage than a radial.

Later Boeing built an XB-39, which was a B-29
with four V-3420 engines, but this too was not
attractive enough to justify production.

Emmanuel Gustin