I don't see how it would over balance the game. It's a bomber, not a fighter. In order to use that big cannon, you'd have to get in close. A below average fighter jock would be able to kill it without too much trouble. Now the La-7 and Spit 16 over balance the game. I'd rather see 100 B-25's cruising around the virtual sky than those planes.
I'd suggest the C model (no cheek guns) for early war, the H model (small perk cost, no formations) and the J model (4 cheek guns) late war. Need the para frags also, those would be fun to play with.