The Luftwaffe frequently installed 20mm cannon in the nose of anti-shipping aircraft to act as flak suppression.
I have seen pictures of all the key Luftwaffe bombers, He111, Do17/217, Ju88, with this set up.
Apparently the machine guns in the rear of the Dornier's cabin provided excellent beam defence, having a good field of fire up and over the wing.
Unfortunately the Dornier suffered from an almost complete absence of defensive firepower from dead astern, contributing significantly to the losses suffered by the type in the BoB.
The Dornier's one saving grace was it's ability to absorb huge amounts of damage and remain airborne, though this did have the effect on morale, with heavily shot up machines making it back to their home base with dead and injured on board.
No wonder the Germans developed Kanal Krank or Channel Sickness.
Karl