On Thu, 14 Aug 2003, Paul J. Adam wrote:

-snips-

The 20mm was a good piece of kit, and seems to have succeeded well
enough to be retained on the P-38 (weren't early versions armed with
37mm?). Similarly, later Cobras went to 20mm rather than 37mm.

The Aircobra I and some P-39Ds (intended primarily for lend-lease)
were equipped with the 20mm cannon. Some of the Aircobra Is ended
up in USAAF service (as the P-400) as did a modest number of the
20mm equipped P-39Ds

All the rest, right up to the final P-39Q model used the 37mm.

Cheers,