i'm pretty sure bombers DO have lethal bombardment, but maybe the units you're trying to kill have more HP than the bombers' ROF. check the rate of fire of the bomber unit in the civlopedia - if it's 3 (i think), then it can only do a maximum of 3 HPs of damage per attack, and therefore would be incapable of killing a veteran unit single-handedly. two bombers definitely should though. good luck!