It looks to me like the B-17 had more forward guns than people to fire them.
According to my count there were two men in the forward compartment
(bombadier and navigator).

Looking at pictures of the B-17F Memphis Belle I see four guns, each in a
separate loophole in the nose of the plane.

Looking at pictures of B-17G's I see 2 cheek guns and the chin turret.
Another picture shows the bombadier's area and he has a cheek gun position
and controls for the chin turret.

Did they switch to whatever gun had a target in range?