Probably the most important target that bombers are actually effective against are TOWNS. TOWNS are often somewhat ignored by friendly fighters (who want to vulch the nearby enemy base instead), so being able to level one with a single plane is a good ability to have.
When flying a bomber for town-busting, I prefer to take one of the Medium bombers and not climb very high....a B-17 or LANC carries more ordnance, but they're so slow that they take too long to reach the target to be worth it. In particular, the A-20 and B-26 are both pretty good for town-busting. Both of these planes have pilot-operated guns, which can be important for destroying town buildings or acks which the bomb drop missed. The B-26 can also take a formation, which may be important to you.
J_A_B