My idea was that the upper gunner has been trying to hit the smaller fighter but is missing it... unable to track it because of its speed and close proximity (I've got fire coming from the ventral ball and tail gunners directed at unseen targets). I considered waist gunner fire but it would have cut across the picture at an angle that cut it in half and maybe spoiled the momentum (anyway, this picture could easily represent a moment in between bursts).
The nearest fighter is firing at an unseen bomber 'off screen' with its nose and underwing cannon.
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