The P-40F with the Packard Merlin did not very much either, was it just a 'draggy' airframe or overweight?
Looks a sweet bird: What did designer get so wrong ? Wing-span short ? Was it just the weighty accumulation of real-neat features ??
{ Apologies for terseness, duty-cat has woken and resumed her enthusiastic sub-editing... }
The design was already largely obsolete. The Allison engine was insufficiently powerful and not yet fully reliable. . The early reduction gear was a problem. Worse, it lacked adequate altitude performance. The single-stage supercharger was too small and too inefficient, having been designed for use as a second stage following a first-stage turbocharger . Fitting a turbocharger into something the size of the P-46 would have been pretty much impossible (look at the bulk of the installations in P-38s and P-47s). Without a better engine, the P-46 did not offer enough improvement over the P-40 to warrant production.
The 'standard' for comparison was the X73 prototype which flew some 4 months earlier than the XP-46. Same engine V-1710-39, same fuel capacity (156gal)
The X73 was basically 10% larger in length, wing span and wing area, 265 pounds lighter with full fuel of 156gal, full load of ammo, armor plate, self sealing tanks (X73 7400 to XP-46 7665).
The X73 flew 20mph faster with full load of fuel/noguns (375 to 355) and farther (900 to 755). The improved cooling system of the NA-73 production Mustang flew 30mph faster and also 300 mi farther - only two months after the first prototype XP-46 and before XP-46 #2.
The shorter wing span of the XP-46 should have been a slight advantage but it used the NACA 23015 airfoil section compared to the high speed/low drag wing NAA/NACA 45-100 of the Mustang.
The fuselage of the Mustang is a 2nd degree conical sections with gradually increasing velocity gradiant all the way to the intake scoop under the cockpit and behind the lifting line. The XP-46 has an immediate 'bump'/disruption with the presence of the first intake scoop, then has the non-adjustable intake scoop immersed in the wake of the boundary layer separation aft of the lifing line of the wing.
I have no idea what the surface quality of the XP-46 ws, but NAA began with high quality, even though X73 was hand made. Tight butt joints, no gaps, flush rivets, but
without the now famous wing surface treatment of the production Mustang.
The production NA73/Mustang I was 30 mph faster than XP-40, despite growing 1000 pounds (8642 vs 7665), and flew 300 mi further with full combat load.
The difference wasn't just Schmued vs Berlin, as Kindelberger formed a project manager team which transformed the Preliminary design parts and asemblies to high tolerance, semi-mass production standard process plans/production assemblies to build high quality with both an eye to minimizing time on the plant floor.
If there ever was a smoking gun condemning Materiel Command's CO, Col/Gen Oliver Echols lack of judgment, the attempts to 'fix' the XP-46 while ignoring the XP-51 is very high on the list. His later attempt to bury the A-36, then the P-51A IMO showed vindictiveness toward NAA for saluting him with middle digit when rejecting his order to build the P-40 for the Anglo France Purchasing Board. When he clung to the XP-75 despite the introduction of the 85 gal fuse ank for P-51B and LE fuel tank for P-38 showed perhaps his stupidity - or perhaps planning for future career after WWII ended?