Which American fighter plane had the highest kill ration in WWII? Hint: it wasn't the Corsair or the P-51. Incidentally, the P-51 was originally equipped with the Allison (GM) engine...a great engine at low altitudes, but sub-par at altitude. The British, being the smart folks they are, fitted them with the RR Merlin, which turned out to be a match made in Heaven. Our military convinced them that lend-lease works both ways, so Packard began building the Merlin here and equipping the P-51 with the Packard Merlin. The P-38 continued to use the Allison, later equipped with a supercharger I believe, and it came into its own later in the war. The P-38 designation for the German side arm as I recall, is short for "Pistol, version 1938".
For those of you that are WWII war bird buffs, I recommend two books: the first, is "Whistling Death", by Boone T. Guyton, the test pilot for the F4-U Corsair. Great book. The second is "Reach for the Sky", by Douglas Bader, a British pilot who lost both legs in an accident prior to the war; learned to fly again and was eventually accepted back in to active flying service in a Spitfire. The history or military channel had special about him not long ago. It is a very inspiring book about a man of great courage and perseverence, also very funny at times.