I will try to organise my thoughts on the subject to be as useful as possible
1. Bomber units are too powerful
In what sense? In the sense that strategy and tactics in the game revolves about them, about upgrading them as fast as possible and concventrate them wherever your main attack goes, to the point of using ground units just to spot enemy units and then retreat leaving the work of pounding the enemy to air units exclusively (memories of an old game, Civil War Generals2, arise)
2. Individual Bomber units are not that powerful
It is the way the players operate them, massing them for attacks against a single enemy unit, what makes them so overwhelming
So, the easiest way to rebalance this, limit the number of air attacks on ground units to a single one per turn. Probably not enough, because you still can mass air units too easily in an small area without supply concerns that would in the real world prevent this happening, but still it is a way to rebalance without modifying combat values.