I can't find a source, but my recollection is that the majority of the U-235 for Little Boy was generated by gaseous diffusion? I know the original process was gaseous-diffusion followed by Calutron and that once gaseous diffusion was improved they shut down all of the Calutrons, and the timing is such that all but the first little boy bomb must have been made by gaseous diffusion, but it's unclear how the fissile material for the bomb dropped on Hiroshima was generated.