If WW2 did not begin until 1942 then the military tech in use would not have advanced much beyond 1939, since most of the development in the OTL 1939-42 timescale was driven by the fact that there
was a war going on.
So the tech development in a war lasting from 1942-49 would probably have been not much more advanced than the OTL 1939-45 development, just another year or so further on.
The A-bomb is the wild card, of course, but I suspect that there would still have been a two-year delay before really getting urgent development underway. A lot would depend on how the ATL war started - and when the USA would get involved.
Tony Williams: Military gun and ammunition
website and discussion
forum