I've done a bit of digging on line and I've found the following figures for the ammo carried on a B17-G:
305 rounds per gun (RPG) for each of the two single Cheek guns
365 RPG for each of the twin Chin Turret guns
600 RPG for each of the Waist guns
650 RPG for each of the twin Upper Turret guns
500 RPG for each of the twin Ball Turrets guns
565 RPG for each of the Twin Rear Turret (sic) guns
At approx 800 rounds per minute that's around 23 - 50 secs per gun. So, firing in short (aimed???) bursts was obviously the order of the day!!!