The only problem with the 75mm gun in the B-25s was the low rate of fire. IIRC the navigator/gunner was at best able to only fire four rounds in a gunnery pass and accuaracy was not very good.
Now the British came up with a Mosquito FB XVIII that was outfitted with a 57mm Molins automatic cannon. The 57mm or 6pounder fired a shell that weighed 7.1 pounds and had a cyclic rate of 60 rounds a minute with a 22? round magazine. A brief history of this Mossie type is at...
Sorry but I don't seem to have a pic of a Mossie with the Molins gun installed.
Regards
alfa6 ;>}