I think the lack of a belly turret is less of an issue than you seem to think. Assuming he does something simmilar to a low yo you, where he trades alt for speed, he will be at his fastest when below you, and therfore you will have the briefest amount of time to damage him. Not to mention that when he passes above you, he can be left hanging there at 0 relative airspeed sometimes as close as D600 while you cut loose with the top turret.
And as to the 30mm issue you brought up, if it can easily kill a B-26 before the B-26 can deal it significant damage with the guns, then the B-24 will be down even faster, and with only 1-2 more .50's firing. Yeah, those .50's can make a difference given a little bit of time, but this is assuming that he is in gun range for a rather brief amount of time (30mm's have a short effective range, about D600 at most against something like a bomber for an average gunner). So if you can't hurt him bad enough to deter him with 4 guns, you probably won't kill him with 5.