Airabonita wing had increased span, chord and dihedral; the fuselage was shortened; cockpit raised
and vertical tail surfaces revised in shape. It wasn't just a tail-dragger
Airacobra.
The 37mm gun was intended as main armament.
Vought won the "contest" with what became the
Corsair.
The XP-39E was powered by the two-stage Allison V-1710-E9 (similar to the engine that powered the P-63)
and the fuselage was 20 inches longer. So, if when speaking of a 'Merlin-inspired' supercharger one means
the two-stage/two-speed type as equipped the Packard-Merlin V-1650, then yeah, it'll be visible as you
couldn't fit it into the P-39 airframe and retain a useful load, a stretch would be sensible. Yes, the
Cobra I & IIair racers had P-63 engine shoe-horned into P-39 fuselages, however these aircraft were stripped to the bone,
and weight/balance issues that would be critical in a service aircraft, were just part of the fun.
A note on terminology, there was and is no 'Merlin' supercharger per se, the various engine driven centrifugal
superchargers used on R-R engines were based on what had become fairly standard supercharger design
principles that had been worked out by engineers and designers in several countries. BTW the French stuck with
a less efficient, and more complicated, home-grown design for the Hispano-Suiza engines after just about
everyone else had adopted simpler, cleaner designs.
Also Allison, if GM had been willing to spend the money to hire enough designers and pay the development cost,
could have come up with a better two-stage design than the one use don the P-63 and P-82 engines, Hell, they
could have adopted elements of Pratt & Whitney's two-stage designs that worked so well on their radials.