Which was the best fighter, the P-51 or the P-38?

We will answer this question from the USAAF's Chief of Staff rather than from the pilot’s perspective.

As a CS, 3 things were of paramount importance:
a) The fighter had to be manufactured in great numbers to equip the fighter groups of a rapidly growing air force.

b) The USAAF had adopted a doctrine of day bombing Germany. The bombers raided Germany in great depth and operated at high altitudes. The fighter had to be able to fight at those altitudes and possess an enormous range to protect those bombers.

c) The aircraft should hold its own against enemy fighters and be as good or better than other domestic fighters.

A) P-38 cost $97,147 USD and a P-51 $51,572 USD (in 1944). After all, it required two engines and more instrumentation than the P-51. So, the USAAF could get two P-51s for the cost of a single P-38. This meant that the P-38 had to be substantially better than the P-51 to justify the extra cost. If the performance was identical, the best fighter for the USAAF would be the P-51.

B) The operational radius of the P-51 by March 1944 was 650 miles versus 585 miles of the P-38 (as per R. G. Davis, 1993). So, the P-51 had a slight edge. Both fighters could reach high combat altitudes.

C) With a nearly identical PWR/WGT ratio (0.39 vs 0.38/P-38) we would expect similar performance everything else being equal (not everything was equal). The P-38 had almost twice as much power, but an appreciably larger frontal area and the P-51 was the faster of the two by 4%. The latter’s rate of roll was better, and it could out-turn the P-38. The Lightning enjoyed better acceleration, better climb, and more firepower. Doolittle requested the RAF’s assistance to evaluate the fighters (and the P-47). Eric Brown reported that the compressibility limit of the P-38 was 0.68 Mach vs 0.78 Mach in the P-51 giving the Mustang the advantage in dive. This was sufficient for Doolittle to request P-51s for all of their fighter groups.
The decision makes sense.

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