The favored attack by fighters against the B-17 was from head on....at best the top turret could have a whack at them or the ball turret as the fighter passed under or the waist, ball, and/or top turret gunners if they passed alongside. The deflection required as the fighters passed alongside exceeded the ability of the power turrets to track them.
The later models of the B-17 added the remotely controlled nose turret and sometimes a .50 cal on either side of the nose.
Formations were modified to bring the aircraft into what was known as a "combat box" wherein the basic group was four aircraft flown in tight formation giving the formation considerable defensive fire power.
Stragglers or damaged aircraft that fell out of formation were at much greater risk than those in a very tight cohesive formation.