GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
Up through the "F" variant of the B-17, the front was lightly armed.True, but the frontal attack was the tactic of the day for the Luftwaffe based on their ability to track and intercept large bomber formations. I think in the greater picture planners would assume that future operations would involve bombers being chased rather than confronted head on as this was evident when gunner positions started going away in the post war years and the last position maintained was the tail gunner.
Someone posted some statistics about Luftwaffe frontal attacks and their effectiveness and IIRC they weren't as effective in bringing down bombers as first thought.
Due to the frontal attacks, the Chin turret was introduced on late "F" types and became standard on the "G".