The Spitfire Mk IV prototype was renamed Spitfire Mk XX and 500 were ordered in 1941, complete with 6 Hispano 20mm cannon. It was renamed for two reasons, 1) a photo recon Spit was given the Mk IV designation and 2) it was decided that Merlin Spits would be Mk I through Mk XIX and Griffon Spits would be Mk XX+ (obviously that was abandoned). That is why there is no Mk XX Spitfire.
So far as I can tell, a clerk saw the order for 500 Spitfire Mk XXs when the Spitfire Mk number was V and, assuming that the order was in error, changed it to an order for 500 Spitfire Mk Vs. If that hadn't happened a Griffon powered Spitfire armed with six 20mm Hispano Mk II cannon would have entered service in late 1941 or early 1942.
Personally, given the targets that RAF Fighter Command was shooting at, I can see no reason to sacrifice performance for that firepower. Two or four 20mm cannon are quite suffient for shooting fights down. As a matter of fact the Spitfire Mk Vcs (four 20mm Hispano Mk IIs) sent to Malta had a pair of 20mm cannon removed because the firepower wasn't needed for shooting down the Luftwaffe and Regia Aeronautica fighters and medium bombers while performance against things like the Bf109F-4 mattered quite a bit.
If we include proto-types (or types just entering production at the end of hostilites) we have to include the J7W1 Shinden, which was armed with four 30mm cannon. It was in production at the end of the war.
Dr Zhivago,
The Do335 has two engines, thus it doesn't count.
