Not ready until after the war in 1945...which really can't be pushed forward more AFAIK because it was already needed to fight the Me262, so was being rushed as is.
Not singling out comrade Wiking in particular here, but this is one of the things that ticks me off about AH. People look at who the Allies were fighting in WW2, and decide that a particular project or piece of technology would have some significant effect on their performance. They then devote a great deal of effort to finding a way to push forward the development of that thing, handwaving away objections by pointing at poor decisions or inefficiency or anything else that hampered it's introduction. This is not, in itself, a bad thing - if it's done plausibly, there's nothing wrong with it.
But the moment anyone suggests the Allies doing something similar, even as a response to the above, there are a thousand unassailable reasons why they couldn't really do any better than OTL, and wouldn't know what to do with it anyway. I just find it really strange that we can give the Germans or the Japanese guided missiles, advanced aircraft or submarines, or whatever the latest toy is a few years early and watch them wreak havoc on the hapless Allies; but for some reason we can't give the Allies Vampire fighters in 1943, or bring out the Centurion in time for D-Day. It's the inconsistency that bothers me most.