The most difficult process for creating the bomb was being able to extract uranium, as well as being able to fuel the nuclear reactor, which was another hardship that occurred. It was found that the only elements which could cause an explosive chain reaction massive enough to destroy cities or cause havok, was an abundant supply of water and P-239. P-239 in simpler terms is an isotope of plutonium which has not been combined yet. To be able to harvest the plutonium, scientists had to use "gaseous diffusion" from uranium ore, they did this process at Oak Ridge. Thankfully the physicist Enrico Fermi, formerly part of the MAUD committee, experienced his first chain reaction which he was able to control. Fermi was performing these experiments in the University of Chicago under the football stadium! Under there he with the assistance of his team stacked 57 layers of uranium metal into graphite blocks to create his reactor.
Scientists first intended to have the plutonium shoot out similar to a gun, but eventually learned that creating an implosion would be much more effective. Implosion would prevent predetonation and also make use of fission (atom splitting). This was just a theory, and the finalization had to be done at Los Alamos with Robert Oppenheimer there leading.