This is somewhat of a difficult question to answer. Mainly, it is because the Mustang is such an increadable plane. It was designed before computers, when designers had no idea how there vessels would preform before the test. Even without computers, the Mustang design team managed to design the plane in just 120 days, something that is unheard of even today. Even without computers to aid in the design and provide projections, and with such a small design period, the Mustang became the best fighter of WWII, and perhaps all time. It was fast enoguh to catch up with all other fighters in the WWII skies, and only surpassed by the German Me-262. Its 6 .50 Caliber machine guns were powerful enough to bring down anything in the skies. It was a very long range craft, able to escort bombers deep into German territory. It has been called "The most aeronauticly advanced pursuit plane in existance."