[Maxisonfire]

But in no book they said that it was this hard to get a B17 like in Il2

You sure? B-17's shot down more fighter aircraft than fighters themselves.

5 hits with Mk 108 30mm Minengeschoss (but you should be a real sniper to hit a bomber 5 times in the same place with mk 108)

"Tests carried out at Rechlin (where most of the Luftwaffe aircraft and weapons tests were done) showed that with a "M-Shell" with 85 grains of explosive, five hits could destroy a B-17 or B-24 bomber..."

http://www.luft46.com/armament/mk108.html