The P-51B is that of Don Gentile. He and John Godfrey were some of the first WWII aviators to use present day ACM, Gentile and Godfrey both flew P-51B's The 51B was the first to get the merlin engine, the p51, P51A and the Apache had the Allison which were mean machines below 12k but didnt do well at higher altitudes. Had the Allison gotten the supercharger the P-38 Allisons had then there would have been a close comparison between the merlin and the allison, however there were significant problems getting all the needs meet so time caused the switch to the merlin and the rest is history.
The P-51B had the four 50 cal. MG's but had serious problems with jamming since they had to be laid on their sides to fit where as in the 51D room was made for the extra set and all could be laid flat which made for much better ammo runs to the MG's.
The 51D had the extra section added to the tail to improve stability since the cockpit interior 65 gallon gas tank upset the planes center of gravity, bigger intake scoop for the radiator to improve cooling, and slightly bigger ailerons, not to mention the bubble canopy which really gives the D model the edge IMHO, all this made it a little heavier, so the 51B had the -3 Merlin where as the 51D had the -7, so all in all they come out pretty close in performance. As is usually the case it will be more often than not pilot skill that would dictate and outcome between the two ponys.
If your lucky you can still find copys of " The Look of Eagles" written by John T. Godfrey, it covers his life a little before and after WWII, but there are a lot of stores about Gentile and himself. They did a lot to make the Mustang famous, by the way thats the name the Brits gave the plane not us, which is fair since they paid for the order
