At some point in late 1944 or early 1945 it is pretty clear to the Manhattan Project that the atomic bomb will work. Certainly the U-235 bomb design, which was not tested as they were certain it would work (the one dropped on Hiroshima). The implosion design plutonium bomb was a more complex issue and was tested to enure it would work. Once it becomes clear this will work I doubt the whole idea will be dropped, although the pace will be slowed somewhat so while you may get a U-235 bomb in 1945, a plutonium one will come later. Given the state of Soviet espionage, what you very likely see is the interval between a US bomb and a Soviet one shrink substantially which will have all sorts of butterflies roaming around. With an atomic weapon never having been used on a city, only in tests, the aversion to their use "its only a bigger boom" is very likely to be much diminished so the odds of their being used in Korea or something similar goes way up.
Absent the US bomb design program and production facilities of OTL the large advantage the USA has over the USSR in the 50s is not there, meaning deterrence/NATO has to rely more on conventional forces against the WP. Also SAC and the Air Force does not get in to the "planes with bombs are the answer" mode at least not until much later. Huge butterflies...