Check out the Junkers-Larson ground attack prototype made for the
U.S. Army in the 1920s. It was basically an all-metal Junkers
monoplane transport with something like 30 .45 Calibre Thomson
Submachineguns firing at vatious angle through the bottom fuselage.
The idea was that it would fly along trench lines at low altitude,
saturating the trenches with bullets. (The opinions of the Gun
Plumber on board who'd have to change 30 75 round drum magazines after
each pass has not been recorded. Rest assured that it would have been
short, to teh point, disapproving, and contained a lock of words that
rhymed with "Duck".)

I could have sworn I saw something on the history channel once about a
modification someone tried on a Canberra... they made a pallet for the bomb
bay and fitted a dozen or so guns pointing straight out the bottom of it...
I think they showed it firing in ground tests, but that's all I've ever
heard about it...