IMO tactics are far more important than firepower and armor when attacking bombers. I can kill a B-24 in an F4U-1A with less than a 1-2 second burst. It's all a matter of where you attack from and where you aim.
If you're flying up his tail, first you're going to take a TON of return fire. Second, if you're firing into his tail there's a good probability your rounds are going into that big, hollow fuselage doing very little damage. Eventually you'll put enough lead in to break something, but you're wasting ammo doing so.
The two best targets on bombers are the cockpit or the wingroots. The cockpit is obvious. Kill the pilot and bomber goes pop. If your aim is good you can cap the pilot with even a half-second burst of 6x.50cal (ask AKDogg about the time he killed three B-24s, I think it was, in one pass with an F4U-1). The wingroots are probably the best, because it's a large target and easy to hit, and depending on the bomber there's any number of components that can be catastrophic to take out.
First, you have the wing spar. Break that, wing pops off, plane goes down. However my favorite is B-24s and Lancasters, and to a lesser extent the B-17, which all place a fuel tank between the inner engine on each side and the fuselage. One good gunnery pass and you'll have him on fire. Leave him and repeat on the other two planes in the formation.
As for approach angle, the best is to position yourself above the formation. You can approach him from the tail, just leave yourself plenty of altitude 'cause he'll probably be spraying you if you're within D1.5k. Rock your wings to pick up his position, and when you're above him roll over and make as near-to-vertical a drop as you can. Aim between the inside engine and fuselage and let it rip.
