It would appear the P-39 was very useful in the low to medium altitude range. In both air to air combat and ground attack roles.
I think the Americans and possibly the British might also have found more use for the P-39 in the ground attack role if a specialized version with an increased range had been built for that purpose. Here is an old what if posting of mine describing a P-39 dive bomber version. Edited to include some authorial rethink.
"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 again restricted by its short range. They were also used there as C-47 escorts in New Guinea. A suitable role for the P-39 as the C-47s were usually flown below 10,000 feet there after crossing the Owen Stanley Mountains.
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 Apache 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 propeller arc for bombs and plumbed for drop tanks as well. Having more locations for drop tanks and even slightly increased wing tankage will increase the range. Some 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 Browning HMGs. The 37 mm cannon would have been a powerful defensive fire suppressant particularly during the vertical dive attack. Especially to exposed AA gun crews.
Other advantages the P-39 would have is the engine being 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 radiator and oil coolers as Bell did with some Photo-recon versions of OTL P-39s. The engine has to be protected as much as practicable as liquid cooled engines and the coolant radiator and lines are more vulnerable then radial air cooled engines.
Using the single stage supercharged Allison engine same as the P-39 is not a problem for low to medium altitude operations. There is little manufacturing changes needed to produce the P-39 dive bomber as compared to the P-39 fighter. Only the modifications to the wing structure and the addition of the dive brakes and the wing bomb/drop tank shackles and fuel line plumbing. One wonders if the USAAF would've given the plane an "A" for attack designator.
This P-39 dive bomber variant would have been a little more robust then the OTL A-36 Apache with a more powerful AA suppressor punch with the 37mm cannon. Much cheaper to build then the 2 man Vultee Vengeance dive bomber and much more capable of protecting itself. I think the Australians in New Guinea would have greatly appreciated such a plane. And maybe the British in Burma as well. 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. And it would still be able to hold it's own against enemy fighters at the low to medium altitude range after dropping its external loads."
Taken and edited from https://www.alternatehistory.com/forum/threads/p-39-airacobra-as-a-dive-bomber.392638/ Please don't reply to the old thread to avoid necroing.
