that's because you know better than to aim at the tail of a b17, if you want to kill it..
right?
and how would you know if you did, anyway?
While back I ran some tests for FSO/Scenarios and wanted to study different attacks on B-17s, amazingly I rolled up behind B-17s with a 190a8 with standard gun package only - and could not kill a rear gunner at 600-1000 out - different converg settings, different attack patterns etc.
Then I realized - buffs have damage points before they go down, same as fighters - simply look at CV ack - buffs fly through all day with few scratches, fighters? poof.
/I ended up never flying 190s, but 262s - I made 9 or 3 clock passes at very high speed and never was dinged once - never again to make a frontal or rear shot on a bomber - period.