Here's a crazy idea:
Bell YFM-2/XP-52 Airamamba? Airanha? I'm open to naming ideas.
Think: missing link between the YFM-1 Airacuda and the XP-58 Chain Lightning
Replace those people at the front of the engine nacelles with two more V-1710s to get four engines in a push-pull configuration (like the Farman bombers and airliners).
Keep the M4 47mm cannons and just condense the P-39's configuration, more like the Hispano-Suiza 12X motorcannons that had been in service for years at that point. Put four .50 cals in the nose and go down to just two crew (pilot and radio operator/flight engineer). Ditch those side blisters and all the various gunners, stick with the tricycle landing gear of the YFM-1A.
You get yourself a four-engine fighter, but you also get about 4,800 hp and can ditch all but two of the crew. Now, it would be impractical and hideously expensive, but you might actually get something in service by 1942, unlike the Chain Lightning.
Best case scenario, you get something like a maintenance-heavy A-26 Invader, but a couple years earlier.
Cheers,
Logan