In early 1939, Hitler becomes interested in researchs on the fission of uranium, that it could be used to produce some sort of extremely powerfull bomb. Such a type of bomb could be used against the Soviet governement in Moscow, quickly bringing about a soviet collapse and secure the place of Germany in europe.
Access to uranium for research purposes partially motivates the abolition of Czechoslovakia.
By summer 1939, a full uranium research program is underway, the start of WW2 and strict secrecy causes some initial problems. Hitler convinces Mussolini to contribute to the project, later Japan join the efforts in exchange for Ribbentrop convincing Molotov that a sovietic invasion of Mandchukuo should be avoided. The defeat of France in 1940 and the resulting confidence of Hitler and the military is a further setback but because the project could also potentially provide much electricity to German industry, it is allowed to continue.
There is a slight boost to the project after the German declaration of war against the US and the continuation of the war against the Soviet Union.
In the meantime, the allies focuses on a plutonium design before the uranium design. In Germany, a shame nuclear projects is set up to attract allied attention, as the allies seems to be aware something is going on, although they don´t know to what extent (ultra).
Between November 1942 and February 1943, the project receives full funding for the purpose of building an atomic bomb.
Plan A: Efforts are also put into a bomber able to reach the east coast of New York, particulary New York. This bomber is to be piloted by a kamikaze, basically make a one-way trip from an aerodrome on the outskirts of Bordeaux.
On July 2nd 1944, the first bomb is completed. Because it is only one bomb and the design haven´t been tested yet, the use of it is delayed enough for the invasion of France to succede, making Plan A impossible.
In October 1944, a 2nd or 3rd atomic bomb is successfully tested. The allied governements are aware this test happened.
Because Hitler suspects a full scale allied chemical retaliation to an atomic attack, he kinda hesitates. Long range bombers are being assembled, to be launched from another Aerodrome near Norway.
As the Red Army encircles Berlin, he begin having second thoughts, before he can order an attack, someone in the SS betray him and steals the bombs, to convince the allies to leave him and his group alone. The US military is able to get the blueprints for the German uranium atomic bomb design.
A german submarine bringing a shippement of uranium to Japan decides to surrender to the allies after May 9th.
The US test íts plutonium bomb but decide to use the already tested German design rather than waste uranium.
3 atomic bombs are sent to be used against the Empire of Japan. The bomber transporting the 1st one goes down and the Japanese brings it to their own research facility in northern Korea, the two others are used against Nagasaki and Hiroshima.
The Red Army launch operation August Storm, in which the Japanese research facilities are seized. The Empire of Japan surrenders.
Of course, the US claimes it came up with both types of atomic bombs, the US governement uses all of its considerable influence to have the cooperation of other allied governements in backing up that claime.
Stalin claimes the facilities where the Germans produced their atomic bombs where throughly destroyed before the Red Army could seize them, that is not quiet the truth.
He also have the Japanese nuclear facilities, with at least one working uranium bomb, intact.
The Soviet-Union is soon to be a nuclear power but Stalin decides to keep the secret for a few more years, because unlike the US, he would be hard pressed to claime Soviet nuclear power is mostly soviet in origins and not basically taken from the hated Nazis, Fascists and Japanese imperialists.
How long would the allied governements be able to keep the secret?