No P-51B until 1944 eh? Not quite.
The first combat unit equipped with Merlin-powered Mustangs was the 354th Fighter Group, which reached England in October of 1943. The 354th FG consisted of the 353rd, 355th and 356th Fighter Squadrons, and was part of the 9th Air Force which had the responsibility of air-to-ground attacks in support of the upcoming invasion of Europe. However, they were immediately ordered to support the bomber operations of the 8th Air Force. The 354th flew their first cross-Channel sweep mission on December 1, 1943, and scored their first victory on a mission to Bremen on December 16. However, inexperienced pilots and ground crews and numerous technical problems limited operations with the P-51B/C until about eight weeks into 1944. From the early spring of 1944, the Merlin-powered Mustang became an important fighter in the ETO.
Now I suppose you will argue it wasn't present in large numbers until '44? We all know how that applies to certain LW a/c too...

As for US designs being from earlier than 1944...
P-51D = P-51B: late '43
P-38L = P-38J-25: early '44
P-47D-25 = P-47D with paddle prop: ??? - I'd guess sometime in 1944 for sure.
F4U-1D = F4U-1A: early '43 - but when did they have the -8W installed?
So saying any 1944 US plane was actually a 1942 a/c is pretty much a total lie, and even 1943 is kinda stretching it, except for the Corsair...
PS: British fighters are the best! Spitfire 21, Tempest II, prototypes like M.B.5, Fury, Spiteful...

[This message has been edited by juzz (edited 12-15-2000).]