to counter the German tactic of head-on attack. SNIP that became their favorite tactic.
It was and developed because the B-17s prior to the G were woefully under-protected from the front with only a single hand held .50 or even .30 cal MG nominally fitted, though there were a LOT of field modifications to fit more .50s around the bombardier.
From the German perspective, one of the other benefits was that, although the aircraft were harder to hit because the frontal area was so much smaller, if you did hit a Fort from the front, the rounds were going to travel all the way through the ship or were going to impact the cockpit, either instance doing way more damage than most strikes elsewhere or from other attitudes.
The Bendix chin turret was specifically designed to provide more punch from the front, but the bombers, be they Forts or Liberators, were always more vulnerable from the front so it was always going to be a favoured attack vector unless the risks for the German pilots were made really, really high.
This looks like an attempt to make them really, really high.
Paul