Interesting stuff.
The P-39 was particularly tragic because it should have been a good plane. As designed, it was blazingly fast, well protected, and was the most heavily armed fighter in the world. As executed, it was kind of fast, fairly well protected, far too heavily armed, not maneuverable, and had a particularly nasty tendency toward irrecoverable spins if pushed too hard due to the unorthodox center of mass. In the interests of streamlining the supercharger had been deleted, basically limiting it to low altitudes. The pilots didn't like them. We gave them to the British, who didn't like them either. (In fact, they sent a lot of them back.) We finally managed to give most of them away to the Russians who were evidently satisfied with them, although they used them as low altitude dogfighters. (They were designed as high altitude bomber interceptors originally, but without that supercharger...) The ones that were not given away were scrapped or used as aerial gunnery targets.
The famed P-40 was completely outclassed by by more modern fighters on all sides. While it was heavy enough to outdive a Zeke, at least one pilot's anecdote mentions a 109 outdiving him in North Africa.
I won't get started on the Brewsters. The only country that had any luck with them was Finland, who used them in a different (and much lighter) configuration than the navy did, against what was most likely not a first rate opponent in terms of experience, training, equipment, and motivation.
In fact, the US didn't field a plane that could compete with a Zero on favorable terms until the F6F arrived in early 1943. I would agree with Tom's assessment, at least as it pertains to aircraft - we were building obsolete planes for a style of air warfare that no one else was interested in, and a lot of pilots paid for this mistake.
To be fair, though, I've heard it said (and I somewhat agree) that Japan was a victim of its own success. The Zero was a pretty good early war plane; the Ki-43 (the IJA's preferred plane) was anything but. It was underpowered, underarmored, and extremely undergunned. Their small arms varied between tolerable and execrable. As Tom mentioned, their tanks were little more than a footnote. There was much room for improvement in both their equipment and their tactics but their early successes were so spectacular that it may have blinded them to their own shortcomings; by the time anyone realized that there was a problem, the war was more than half over and it was probably too late to make a recovery.
