The only 30mm somwhat available back then was the giant MK 101, which, already considered obsolete, was only fitted to a few Do 24 seaplanes and later as a external gun option on the HS 129 tank-buster.
I'm not aware of any Do 24 use (although I always welcome enlightenment) but the MK 101 (or MG 101 as it was known at that time) was fitted in a belly pod to the Bf 110C-6 used in small numbers in the Battle of France, the Battle of Britain and IIRC in North Africa. One was shot down over England in the BoB and I've read the RAF report on the wreck, which described the gun and its ammo supply.
A pair of MK 101 were also fitted in a chin turret to several He 177, apparently with the intention of engaging US bombers being ferried over the Atlantic, but this never happened.
Tony Williams: Military gun and ammunition
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