I always felt the P-51 and P-47 had two distinctly different missions. The P-51 for high altitude bomber stream support and one on one combat with the enemy fighter aircraft.
In practice, that's how they turned out but there origins were a bit more complicated
The P-47, as I understand it, started out as a fighter competitor alongside the Curtiss XP-46 (never entered service) known as the XP-47A. The goal seemed to revolve around producing lighter aircraft which incorporated some of the best features seen in European aircraft (more streamlined, lighter weight it would appear) in both aircraft.
The XP-47A was not well received it would appear by the USAAC because it's wing-loading was a bit heavy, and Alexander Kartveli seemed to favor radial engines over the V-1710's that were proposed because of their sturdiness. There were also some combat reports from Europe that came back indicating a need for an aircraft that wasn't just fast, but rugged and heavily armed (I'm not sure if they realized that at this point that the RAF's Hurricane and Spitfires both had 8 x 0.303 as a means of putting more rounds on target, and the Me-109 had 20mm in the wings). With experience on the P-35, the XP-41 (a P-35 with a twin-stage supercharger and streamlining), the P-43 (had a turbocharger), they built around the R-2800 with elliptical wings like the P-35, and a turbocharger like the P-43 had giving good altitude performance, and produced "a dinosaur" but one with good proportions.
This aircraft was called the XP-47B, and while it had a variety of problems early on (the canopy opened sideways, and wouldn't always open right for bailout, problems with the control surfaces causing aileron snatch, and poor tuck-under response), they would be worked out in the P-47C production run.
The P-51, from what it appeared started out as an export fighter for the French and, if any attempts were made to build them here in the US, it seemed unsuccessful. This was around 1939. By 1940, the British wanted to import more fighters and since the P-40 performed fairly well, they went to Curtiss to buy some, and then went to North American to buy more. They didn't want to build it since they had a better design that they could get built and flown faster. They asked them if they could build it to a specified timetable from when he contract was signed, and it was agreed upon.
The early designs used the Allison V-1710 and a single-stage supercharger, and were able to achieve around 378 mph which was better than the P-40 could achieve at the same weight on the same engine. The belly mounted radiator was variable geometry able to vary the inlet area, and was actually a success (The P-40 originally was to have a belly radiator, as was the XP-46, and the Hawker Tornado, neither worked), though the P-51A soon incorporated a fixed radiator that had a large splitter. Speed went up slightly past 400 mph at 15,000 feet, and was still around 378 by the time 20,000 feet was reached, making it about as fast as the Spitfire VC except with twice the range.
Both the P-47 and P-51 were used for escorting at different points in time, the problem with the P-47 was basically that
- It's early range wasn't all that impressive to start out with
- The desire to use drop-tanks was either stubbornly opposed or not pursued to the degree it should have
- The P-47C on had a ferry tank, but it didn't work above 10,000-14,000 feet as it was not pressurized -- turns out it's altitude limits were basically the same as ours
- When drop tanks were procured, they didn't procure them in large enough numbers at first to be useful
- The maximum dive speed was 0.72 mach which was less than the Me-109 and Fw-190
The P-51's naturally had good range to begin with, though it's critical altitude was a little bit low. Contrary to the story that the low altitude was a shock to the designers, that's actually not true at all, the performance of the engines at altitude were well known facts already. The high performance of the aircraft made it a good candidate to fit the Merlin 60's.
Some changes had to be made including the exact shape of the nose, the carburetor intake was placed underneath the engine instead of above as before, the greater engine power resulted in a four-bladed propeller, the twin-stage supercharger used an after cooler, which demanded modification to the aircraft's belly-radiator to dispel the heat produced by it, and that made it into a substantial airplane. The P-51B could achieve 447 mph at 29,800 feet with the V-1650-3, the -C used the -7's if I recall which had more overall power but was geared for lower altitudes so it reached peak performance around 24,900 feet, but as a result of having more power, even as it went above the critical altitude there was still enough to achieve a decent rate of speed. The use of drop-tanks of increasing size helped drive up the range, and when the center tank was added on the aircraft, it became truly remarkable (though the aircraft was almost neutrally stable when the tank was full).
It was capable of diving at Mach 0.75 which put it similar to the Fw-190 and Me-109 in theory
The P-47 provided great support for our ground troops.
Yeah, it was extremely rugged and turned out to work magnificently in that area. It's ironic that it was originally designed for high altitude