[CCIP]

Historians hate what-ifs for a reason. You can argue almost anything by changing the facts/circumstances, when in fact nothing can exist without them...

My own view is that the XXI, even if it came around earlier, would still be too little too late. Even by 1942 the Germans were already in fact doomed, if only in the most crucial theater of war - the Eastern Front, which would have been impossible to stop with submarines.

As far as the XXI itself, its earlier introduction would have just accelerated the development of countermeasures, of which the allies had many. While it certainly would be a much bigger challenge, it was still fundamentally susceptible to many of the weapons the allies already had in numbers - hedgehogs, squids, downward-sweeping sonar, MAD, sonobuoys, FIDO. I think there's no question there would be a serious shock - but that shock would be the kind that, let's say, the success of Operation Drumbeat was, not by any means the kind that Hiroshima and Nagasaki were. It was a weapon with obvious and counterable weaknesses. On top of that, Blair quite correctly identified many flaws, teething troubles and design failures it had. The net result would be that they would run wild for about half a year, and then the allies would catch up. They already had all the technology needed to kill these boats, it was just a matter of developing it further.