The Atom bomb initially came into America's thoughts after Einstein had told Roosevelt about the Nazi's attempts at purifying Uranium 235.
Shortly afterwards, the Americans launched a secret project known as the Manhattan Project with the goal of producing a viable atomic bomb.

The most complicated scientific problem at the time was being able to produce enough 'enriched Uranium' to sustain a chain reaction. The extraction process produced a low rate of conversion from ore to metal and even when this metal was extracted it was over 99% Uranium 238 which was practically useless for an atomic bomb. To complicate matters even more both Uranium 235 and Uranium 238 are isotopes of Uranium which meant they were virtually identical in their chemical make-up thus making separation extremely difficult.
To solve this problem a massive enrichment plant was built at Oak Ridge, Tennessee using the principles of gaseous diffusion and Ernest Lawrence implemented a process involving magnetic separation of the two isotypes.
A gas centrifuge was then used to further separate Uranium 235 from Uranium 238. With all these processes in place it was time to test the entire concept behind Nucear Fission.