I'm sorry, but if there is really going to be a Nazi bomb before 1940 then this TL will need to be moved to the ASB section. It doesn't matter how many resources get thrown at the problem - the basic science simply isn't there to go from a science fictional concept to a working device in 8 years. To take a couple of problems - nuclear fission wasn't even theorised until 1933 and wasn't demonstrated until 1938 (and a sustainable one wasn't demonstrated until 1942). In addition there are several additional problems unique to a Nazi programme -
Deutsche Physik. Note that Heisenberg (without whom you can probably forget about a Nazi bomb in any reasonable timetable) was almost arrested by the SS for refusing to stop teaching "Jewish physics" and was only saved by the astonishing coincidence that his mother went to school with Himmler's mother and was able to get the old girl to bend her son's ear. Deutsche Physik was eventually put on the back burner OTL, but I can see a Hitler obsessed with SF concepts but knowing little proper science giving it more of a hearing.
OTL the surviving German nuclear scientists after the war tended to explain the lack of progress on the German programme as being due to ethical concerns on their part over giving the Nazis a bomb. Even if you view this as ex post facto rationalisation, the alternative excuse - they were simply incompetent - needs to be considered.
Resources. OTL even the USA could only run one programme of the size and scale of the Manhattan project. the closest the Germans got to this OTL was the V-weapons project (which IIRC was something like a third the size of Manhattan, in a much smaller economy). If the Germans try to run a bomb project alongside a V-weapon project they will simply bankrupt themselves and won't have anything left for conventional armament.