The B is almost identical to the C other than point of assembly. But I have seen some resorces state that the C was slightly faster.
P-51B (NA-102/104) production version of XP-51B,
6,840lbs empty, 9,200lbs normal, 11,200lbs max
1,620 hp Packard twelve-cylinder Vee liquid-cooled Merlin V-1650-3 twelve-cylinder Vee liquid-cooled Merlin
388 mph @ 5000 ft, 406 mph @ 10,000 ft, 427 mph @ 20,000 ft, 430 mph @ 25,000 ft, 440 mph @ 30,000 ft
1.8 minutes to 5000 ft, 3.6 minutes to 10,000 ft, 7 minutes to 20,000 ft. Service ceiling 42,000 ft
Four 0.5-inch Browning MG53-2 guns in wings, 350 rounds each gun, and 280 rounds each outer gun
NA-104 wing hardpoints strengthened for 1,000-lbs bombs
Final 550 aircraft added 85-US gal rear fuselage tank (P-51B-7-NAs), also retrofitted to some earlier aircraft
Green camouflage paint removed later in production run to reduce weight/drag
1,988 built, 25 to RAF
Mustang Mk III British equivalent P-51B/C (274 P-51Bs and 626 P-51Cs)
British addition of "Malcolm Hood" or "Bubble" canopy on Mustang III improved visibility, retrofitted to many P-51B/Cs
P-51C (NA-103/111) Similar to P-51 B but built in Dallas TX , (not Inglewood CA)
6985 lbs empty, 9800 lbs normal, 11,800 lbs max
1695 hp Packard twelve-cylinder Vee liquid-cooled Merlin V-1650-7
395 mph @ 5000 ft, 417 mph @ 10,000 ft, 426 mph @ 20,000 ft, 439 mph @ 25,000 ft, 435 mph @ 30,000 ft
1.6 minutes to 5000 ft, 3.1 minutes to 10,000 ft, 6.9 minutes to 20,000 ft. Service ceiling 41,900 ft
So in AH the B=C and the D=K (Although the K had a Aeroproducts propeller, but was changed in the field for reliability/durability)