Aiming center of mass is fine against fighters, you're bound to hit SOMETHING important (I don't aim for individual components against fighters, I'm not a good enough shot for that). Bombers are entirely different. WHERE you shoot is more important than how much you hit and with what.
As Messa said it depends on the target, although hitting the cocking in a head-on shot works for all of them. I also find a vertical drop on the wingroots is effective against all BUFFs, though as indicated the B-24 is most susceptible to this.
But you have to concentrate your fire. Waving your guns and scattering hits all over isn't going to do you any good. Keep your attack run steady and focused on one point. Just break off before you run in to him.
And DON'T sit on his tail. It takes a lot more fire to cause enough damage to bring him down that way than it would if you make a high-angle pass that gives you a clean shot at one point.