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Of all the aircraft I would choose the B24 as the biggest game changer as for the reasons set out above . An often forgotten part of the war is how the 24 changed ASWAgree with all the above but can I add 1 more the VLR (very long range) Liberators that closed the Atlantic gap and neutered the U boat threat. No Liberators, possibly no D Day or at least a much later one.
The KI43 was a formidable opponent and instrumental in the early Japanese success and it did all that with only two 12.7 MGs. However, the Zeke was unique in the world in that it had a combat radius of 300 miles off a carrier. That was twice that of the F4F.
Thorlifter, I doubt the F4F or P40 ever had a kill ratio of 3-1 over the Zeke. Lundstrom, whose books are heavily researched says that the F4F in November, 1942 had a roughly even ratio with A6Ms, which was chiefly due to superior tactics by USN pilots. I doubt the P40 did as well. The FM2s later in the war may have done a little better as the quality of the IJN pilots declined.
Not strictly the same, but I put forward aircraft equipped with the cavity magnetron - or at least centimetric radar. Over the North Atlantic RAF aircraft fitted with ASV (Air to Surface Vessel) radar (this is what really tipped the balance in favour of the Allies against the U-boats) were able to intecept U-boats long before the U-boat was aware it was being stalked. Over Enemy territory, British heavy bombers equipped with H2S ground mapping radar were able to make their way to their targets and bomb them more accurately than before, British night fighters fitted with AI (Air Interception) Mk.IV radar had the advantage over German bombers over the UK, and last but not least, post-war, the humble mcrowave oven - melted cheese on soggy bread, anyone?