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In SP missions, the Bombers are AI and generally set to fly flat out without suffering the problems of CEM.
Also, the default loadout is not much use as it uses Observer rounds and Tracers. The observer rounds do nothing except give you the impression you are causing damage but cause none whatsoever. Change your loadout - plenty of threads on effective loadouts can be found in the forum by doing a search.

The rest is down to technique and gunnery practice - knowing the blind spots of each of the Axis Bombers and where to aim, etc.