Sure, they could drop one. On China.
They bombed the US mainland TWICE, so far as
I remember. Both submarine based planes, unable
to carry the necessary payload. And the subs were
pretty much destroyed by late war.
But, do you have
ANY reason to believe that they were close to
developing a bomb? Certes, I don't remember
ever hearing anything along those lines, so it
must have been a recent revelation.
There was some fear about Germany, but they
never were as close as was thought.