Drawing by author Taylor Anderson used with permission
SPOILER WARNING Straits of Hell and beyond.
PB-5D "Clipper" has stacked engines and mounted defensive weapons with hardpoints for ordnance. Taylor included as a teaser with other art. Note stacked engines for more power. Taylor wrote "I'm tossing you a preview of the new "D" Model that will be briefly alluded to in "Straits." The D model uses the same airframe as the B, but stacks the radials, returns to the fixed wingfloats, and boasts hardpoints under the wings."
Note the mounted Browning Machine Gun M-1917 copies for defense.