The P-51D-5 is a Tier 4 american aircraft. It can be unlocked by purchasing one of the P-63 kingcobra variants and one other tier 3 aircraft(1.37), and then grinding alot of xp before buying it for ridiculous sums of lions.
As with many american fighters, the p-51 lacks in turning but works well in dives while dogfighting. Againist tempests and FW 190s how ever, go for the head on attack and spray and pray. The P-51 does not hold up as well as one would like, with 2 or 3 20mm or higher caliber hits in the aft section between the cockpit and the tail rudder will usually result in seperation of those pieces, and scores an easy kill for the lucky pilot who pulled off the shot.
The P-51 excels in the ground attack role as well, as do many tier 3 and higher aircraft. the P-51 can either carry 6 127mm rockets, 2 1000 lbs bombs, or 6 114mm rockets. If you choose for the multi purpose role, use either 127mm or 1000 lbs bombs, as the hard points for these weapons will give minimal drag and allow you to engage in dogfighting. the 114 mm are actually bazooka launchers mounted under the wings, and create enough drag that dogfighting is not viable to do with these equipped.