alternatehistory.com

The P-39 was a reasonable good fighter providing they were operated below about 15,000 feet and taking into account its short range it still achieved success flown by Russian/Soviet pilots against the Luftwaffe in the low to medium altitudes where their air war was largely fought.

When not being used as a stop gap interceptor it provided good service to the Americans in the New Guinea and Solomons campaigns though P-39 operations were restricted by its short range.

The P-39s effectiveness as a fighter-bomber could have been increased with relatively minor modifications mainly to the wing. Removing the 4 .30 machine guns from the wings and using the space for fuel tankage would have increased the range maybe 30 to 40%. In the dive bomber role the .30 guns are not useful.

The wing structure would need to be beefed up to take the loading the A-36 style air brakes would impose during the vertical dive and to withstand the high-g pull outs. Bomb shackles could have been installed just outside the propellor arc for bombs and plumbed for drop tanks as well. Much of this is similar to the modifications done to the P-51A wing to produce the A-36 Apache except for the removal of all the wing guns.

What the P-39 dive bomber would have had that the A-36 didn't was a 37 mm cannon in the nose as well a 2 .50 cal Brownings. The 37 mm cannon would have been a powerful defensive fire suppressant particularly during the vertical dive attack.

Other advantages the P-39 would have is the engine better protected in its' mid plane location and the coolant radiator and oil coolers buried with in the fuselage under the engine. This is especially so if the extra armour is installed around the rads and coolers as Bell did with some Photo-recon versions of OTL P-39s. And perhaps a little more armour in the nose and lower nose area to cover some gaps though ofcourse weight will be a main consideration. The engine has to be protected as much as practicable as liquid cooled engines and the coolant rad and lines are more vulnerable then radial air cooled engines.

Adding a trapeze type bomb mount to the centre line mount so the bomb can clear the propeller completes the transition from fighter bomber to longer ranged fighter dive bomber. Using the single stage supercharged Allison engine same as the P-39 is not a problem for low to medium altitude operations.

So the P-39 based dive bomber would have been a little more robust then the OTL A-36 with a more powerful punch with the 37mm cannon. Much cheaper then the 2 man Vultee dive bomber and much more capable of protecting itself I think the Australians in New Guinea would have greatly appreciated such a plane. Of course it would not need to have been used only in the dive bomb role but all fighter bomber type operations would have been possible as well.
Top