Guns on a bomber have two purposes.
1. Defensive fire against enemy fighters.
2. Offensive fire in a strafing attack.
Notwithstanding what some people will say about Allied 4E, AE bombers don't dogfight/sweep enemy fighters. Plus bombers never engage enemy bombers on "CAP" as that air mission is not available for bomber units.
The primary use of guns on bombers is therefore the defensive role. When a fighter attacks a bomber, whether it approaches the bombers head on, from underneath, from behind or from one of the sides, is largely a random call. Of course if the fighter is climbing to meet the bomber it is most unlikely to be attacking the top side of the bomber or conversely if patrolling above the bomber, it is most unlikely to be making a belly attack.
There is therefore a random chance that a B-25G will be attacked head on and if that is the case the 75mm cannon indeed become an anti-aircraft weapon.
You cannot strafe from 7k+ feet. Strafing of enemy LCU is an option but the real use of those cannons offensively on an attack bomber is on a low level (1k feet) anti-ship strike. They help to suppress enemy flak and can inflict large holes in ships, particularly non combat ships.
As usual with AE, applying the historical tactics is usually the way to go. Mind you, because of the abstractions which are inherent in the game, one cannot slavishly apply historical tactics without due regard to the specific game tactical situation and the necessary compromises resulting from the abstractions.