x1 37mm & a few .50's or 30cals to the same area will get you the effect your looking for. The p-39's 37mm cannon is kind of weak when compared to all other 37mm, sure the field guns have 37mm, but the ammo itself is nearly 1/3rd larger with more power, thus more kinetic and deeper explosive penetration. Even the 37mm single shot cannons the army used had larger rounds.
Also consider the angle of attack, it is possible for rounds to deflect a bit off of a target, and not allow for deeper explosive penetration.
Its all in the numbers,range, angle of attack and such all matter for the 37mm on the p-39 because its SOO close damage point wise for doing all or nothing. With bombers, aim for the pilot, tail's or the inner wing where it meets the wing root. You will have a fireball.