A Chin Turret was added to the B-17 because the placement of the guns in earlier versions of the B-17, left the airspace immediately in front of the aircraft largely uncovered.
German and Japanese pilots soon realised this and mounted head-on attacks on the US bombers in the hope of killing or incapacitating the pilots, navigator and bombardier. All whom were clustered in the noses of their aircraft, and who were essential to its mission effectiveness and in the case of the pilots, the aircraft's very survival.
The remote control barbette “chin turret” was intended to eliminate, or at least mitigate, this vulnerability. The bombardier operated the chin Turret.