The P-51D Mustang is an air superiority aircraft, armed with six .50-cal machine guns.
The P-51 was originally designed as an export fighter for the British Royal Air Force; when North American Aviation was offered a contract to license-build the Curtiss P-40 Tomahawk for the RAF in 1939, NAA responded that in the same time they could design and produce a superior aircraft using the same Allison engine. NAA flew the prototype in October 1940, six months after the contract was granted. As part of the deal, two aircraft were provided to the US Army Air Corps for evaluation.
The Allison engine lost performance above altitudes of 15,000 feet, relegating the early P-51A (Mustang Mark I/II in RAF service) to secondary roles such as ground attack and tactical reconnaissance (the former leading to the development of the separate A-36 Apache dive-bomber variant for the USAAC). On the plus side, below 15,000 it was a fast, maneuverable aircraft. The Mustang's career really took off when the RAF converted five aircraft to Mustang Mark X specifications using the Rolls-Royce Merlin engine, supercharger, and the four-bladed propeller of the Spitfire Mark IX. Now able to maintain its performance to altitudes of over 30,000 feet, the Merlin-engined P-51 entered US service as the B-D models. The addition of "wet" hardpoints for drop tanks, on top of a large internal fuel capacity, gave the new Mustangs the range to escort strategic bombing raids all the way into German territory. This resulted in most US fighter units in Europe converting over to the Mustang by the end of 1944.
The Mustang's six 50-caliber MGs lack the devastating punch of the Fw-190A's 20mm cannons, but the faster rate of fire compensates for this. The P-51 is one of the most dangerous air-to-air combat aircraft in the game, combining impressive speed and agility with firepower adequate to end any aircraft. It's high fuel capacity also allows it to patrol over a battlefield longer than any other fighter in the game.