Re: B-17, B-24, B-29, or Lancaster?
Got it from The Encyclopedia of WWII, it's at my parent's place so I can't tell you the editor's name or the publisher. It says some Mitchells had the crew position in the nose removed and a 75mm(?) howitzer mounted in the nose in an abortive attempt to create a tank-buster. It didn't work, maybe they should have mounted it sideways like in modern gun-ships.
Here's a link, they say it was a 75mm. and was used to sink ships at a rate of fire of three rounds a minute.
Here's another site and a pic from that site, notice the large aperature on the nose.
Now DO NOT question me again. (KIDDING)
Quaerite Prime Regnum Dei~ Official motto of Newfoundland
If guns are outlawed, only the government will have guns. Only the police, the secret police, the military, the hired servants of our rulers. Only the government --and a few outlaws. I intend to be among the outlaws. -- Edward Abbey in Abbey's Road, p.39 (Plume, 1979)