Sorry, but your submission has been removed because we don't allow hypothetical questions. If possible, please feel free to rephrase the question so that it does not call for such speculation, and resubmit Otherwise, this sort of thing is better suited for /r/HistoryWhatIf.

This blog post covers your question very throughly with a lot of primary quotes.

The initial target was always intended to be Germany. But early on it was apparently considered a weapon of retaliation, to be used in response to a German atomic weapon. It wasn't initially viewed as a first strike weapon.

Although as early as 1943 there were discussions about dropping it on Japan first. If for no other reason, they expect the Japanese would be less likely to be able to reverse engineer a bomb than the Germans. By 1945 the planning group had completely switched to Japan as the target. The war, it was believed, would be over in Europe before the bomb was ready for use. But we do know that as late as Dec 1944 there were still discussions over how quickly one could be made available, whether it could be used to support The Battle of the Bulge, etc.