Have the Manhattan Project discover the Zippe centrifuge. This was tech that could have been built with early 40s technology; the Manhattan Project just got unlucky and missed it, and instead went with thermal diffusion etc. Zippe is today the cornerstone of most uranium enrichment, because it has significantly lower capital costs and energy requirements than Manhattan-style enrichment. I'm not sure exactly how much that pushes up the bomb, but I would bet it would buy at least a few months.
If they went all out, they could probably have the bomb by '44 (late '44, mind you) even with OTL techniques, given that work didn't start in earnest until late '42 or so (that's what I've read, anyways). Put them together, and...how does clearing the beaches of Normandy with
tactical nuclear strikes sound to you (of course, they'd need to have British-based B-29s for that)?
Of course, far more likely would be replacing the firebombing of Dresden, etc. with the nuking of Dresden, etc. This would be much more effective at destroying industry, probably, than the OTL strategic bombings. It would also make the China-based B-29s more effective, and would tend to make people focus on that, perhaps? Could Silverplate bombers based in China reach Japan?
Somehow I suspect that ITTL the war would be over by Christmas '44...