But you have boosting techs for subs and fighters, both cheaper than bombers. Maybe making HBs a 5 attack, 2 defense unit who gets a +1 for strat bombings would be enough.
Anyway, with OOB rules, as Subotai said, I’m more afraid of HB affecting naval combat. I’m not sure about this, maybe reduced cost of ships (even more with shipyards) can counter HBs.
Another item is that HB could be too powerful in Atlantic and Mediterranean with so small distances, but Pacific is another story, it has larger distances. And you need some fleet for defending your trannies and take that yummy islands. Figs can defend fleets, HBs not.
Of course, a first round HB tech could be very powerful, but chances of it with 1 dice are 1/36, less than 3%, and there are few bombers on board. Maybe is too heavy forbid techs only for so slim percent. Even so, I prefer a mandatory tech with less powerful HBs than no tech in all. Techs give a special taste to the games