A bit of random speculation, after pondering the Oppenheimer movie:
One key scene in the film makes clear how the persecution/extermination of Jews by the Nazis and European Fascists in general drove Einstein, Szilard, Frisch, Teller, Fermi and many other scientists of Jewish descent or relations to the U.S. and U.K., thus helping to cripple the German nuclear program and boost the Manhattan Project to success. In his TL191 series and the Crosstime book Curious Notions, Turtledove details worlds in which the Nazis don't arise, and so the majority of these scientists remain in Germany or Europe overall, forming the core of the Central Powers brain trust; therefore, the Kaiser gets the bomb first in both cases, because the Germans don't persecute Jews to the extent that Tsarist Russia or AF France do (they're "too civilized for that sort of thing", as I recall one character saying).
Setting aside the plausibility of that for the moment, presuming a scenario where the U.S. doesn't have a large number of specialists from Europe, Jewish and otherwise, on hand to complete an atomic bomb project in the period 1930s-1950s, who would be the likeliest people approached for such an undertaking? Oppenheimer is an obvious choice, on the presumption he's still born in this TL, and Feynman is another possibility, but who else? And if the OTL emigre specialists are working on an A-bomb for Germany in such an ATL, for one reason or another, what are the odds they succeed before the ATL Manhattan Project?