IRL bombers needed gunners because leaving them in a situation where they would be completely helpless would have been a disaster. Escort fighters couldn't cover everything at once, particularly if the intercepting enemy had an altitude, and therefore energy, advantage.
Now, in the game all this goes out the window and you're better off stripping the bombers from their gunners and adding something that increases the air defense stat instead. Gunners, referred to only as turrets in-game, only boost the air attack stat. Somewhat ironic, when you consider how the stats are named.
Oh and while you're at it, don't give your bombers anything that gives them air attack, except maybe in a situation where you don't have enough thrust for anything more useful. Giving them fixed MGs or cannons meant to combat other aircraft provide nothing against ground or naval targets whatsoever. Strafing doesn't exist in the game. The game hates compromise designs, and you're best off leaving the air war to dedicated air superiority fighters, while CAS with as high a ground attack stat per production cost as you can produce, help win the ground war.
EDIT: Although with strat bombers you might want turrets if bombing states your fighters can't reach. But that's a niche scenario, and mostly only relevant to MP.
