The B was built in Inglewood, Ca. and the C was built in Dallas, Tx. That's the only difference between the two. The Malcom hood for the B/C was a field modification undertaken by the British and some were used by the USAAF. The D had the bubble canopy and 3 guns per wing instead of just 2 guns per wing in the B/C models. You will also see the dorsal fins installed on some B/C models, but the D's had them installed. That's the major difference between them.