The early P-36 and P-40 aircraft were totally lacking in both armour and self sealing tanks so they were no better than the A6M at that time. They were first fitted to P-36 and P-40 aircraft ordered for Britain and France. I suspect the first P-38s and 39s may have also lacked those items although the British and French demands and experience were starting to penetrate the US design philosophies by the time Pearl Harbor happened.
As a general rule of the thumb US fighters that have a letter designation of "C" or lower (like P-39C) were judged NOT combat capable at some point in 1940 or early 1941. This made figuring out which planes were combat capable (or capable of over seas deployment) much easier as the people doing the counting didn't have to refer back to spec sheets all the time for each type of aircraft. However this didn't last long. By 1942 all of the older fighters had been transferred to flight schools or ground schools or shipped off to Panama. And the the newer planes (P-47 & P-51) were pretty much combat capable (not P-47Bs) right from the start.
The "protection" gets a lot of the blame. The American planes were built to higher strength standards to begin with. This did allow them to stand up to more damage (mostly).
1930s Navy planes had unlimited dive speed for instance. To be accepted the plane (fighter or dive bomber) had to sustain a "terminal veleocity dive" Plane was dived to a speed where the force of drag equaled the force of gravity and the plane would not go any faster. This got rather hard with low drag mono-planes and the F4U was either the first or one of the first that did not have to do this (they calculated how up they would have to start and how much room they would need to pull out and things started getting a little dicey).
And nobody wants to blame the sacred cow of American armament. The .50 cal machine gun. When you stick 2-3 times the weight of guns/ammo one 6000-7000lb plane that you do in another something (or several somethings) in the performance takes a hit.
As for the P-66 looking like an A6M all I can say is that you must think the Hurricane looks like the Mustang.
Sure.
P-66 and A6M, both low wing mono planes with retracting gear, both radials with transparent canopies, Better than close enough for CGI/AI work
Hurricane and Mustang, both mono planes with retracting gear, both have pointy noses and transparent canopies. Better than close enough for CGI/AI work