Aim points

[Soviet Ace]

Engines. Always aim for the bomber's engines. You get enough rounds in quick enough, or just one really good lucky burst, and you'll have a B-17 or He-111 going down in flames for sure.

[1.JaVA_LGorrit]

Yep, engines or cockpit if your angle of attack allows it (especially on a head on).
Else you can mess up the steering controls (elevator), or fuel tanks in the wings.
Shooting at the fuselage will not bring a bomber down.