The P-51 (not P-51A, the one BEFORE the -A) had 200 made with 4x20mm cannons. These were planned to be sent to England, but the US took posession of them. They used them mostly for straffing, I think. They had the Allison with a low-alt power curve, so it's got less power and lower alts than the P-51B/D.
It's been requested a few times over the years, but chances of seeing it in-game are pretty slim. It saw minimal action, compared to the MG-armed Allison P-51As that followed.