It was a specification for an interceptor with cannon and machine guns, the idea was purely American, but it was also a bit of a con job by Bell as they promised a 400 mph fighter, and when it went into production they had changed the supercharger and it was not as good as it should have been. The RAF kept some of their order, but most went to the USSR, and they were perfect for low altitude work as they were very maneuverable...
I thought the main drawback of these was their lack of maneuverability? When facing off against Bf-109s and Fw-190s I mean?
I recall the Russians used these against trains as well. Blam. Blam. Gone.
One thing that always kinda bothers me in the flightsim is how close to the ground the Russian planes operate usually. Well, if I'm in a Messerschmitt I don't have any problems with it of course... They kept pretty low, didn't they?
The roll rate on the P-39 was phenomenal - 70% better than a Zero, Hellcat, Wildcat, or Lightning, so it did well at low level, but that was not what the RAF wanted... And the Russians were very successful in using it as a fighter and a train-buster, but that big cannon fired slowly, so it was ditched by the RAF in favour of a 20mm...
I have hardly ever flown a flight sim, but I'd guess you would want to keep as high as possible in your Messerschmitt then swoop down to use your superior speed, and try to avoid the usual twisty-turny dog-fight...
I figured I'd try and use all settings on "realistic" for once (except for bullet ballistics, that's still too hard for me) and put myself to the test against what I believed to be a maneuverable plane, a regular P-39 with experienced AI, with me in an Fw-190 (because of its firepower). But after dodging its initial head-to-head attack (just in time, too) I had no problem turning after it, despite its frantic dodging attempts. The outcome was a bit of a surprise to me... Later I read somewhere that P-39 were kinda outclassed by the German fighters of the time.
Still, that was AI, and not a real-life situation. I'd be toast fer sure. Besides, the "outclassed" might be more of a speed thing, like you mentioned. And I must say that is an impressive roll rate. It looks like a nicely balanced fighter, but it needs a diet and more power, I guess.
On Messerschmitts: definitely! I always diverted my flight (if I got to lead one) away from the assigned flight path, always in a very steady few degrees climb, so when I'd arrive at the action I'd be at maybe 25,000 ft high. Worked very well against P-39s. Dive down like a madman, take one out, a wide loop upwards to bleed off speed and gain maneuverability, and there you are, at the perfect spot to dive on the next one and repeat the trick once or twice. And since the AI-driven P-39s are climbing to try and get a shot, the altitude creates the space suited for a 109-favoured turning dogfight as well. I must note, these missions take place somewhere around 1943-1944, and the Bf-109s of that time were a far cry from the "Emils" from the battle of Britain. Such a fine plane.
Again, it probably wouldn't work against human opponents. They'd know better than to be lured into bad situations. And I'd probably know better than to recklessly dive on one.