Until the atomic bomb is tested, its effectiveness will remain in doubt. The world had never seen a nuclear explosion before, and estimates vary widely on how much energy would be released. Some Los Alamos scientists continue to doubt privately that it would work at all.
Only enough weapons-grade uranium was available for a single bomb, and confidence in the gun-type design was high, so on July 14, 1945, most uranium bombs ("Little Boy") made their journey west across the Pacific. started. Its design has been thoroughly tested.
A test of the plutonium bomb seemed important, however, both to confirm its novel detonation design and to collect data on nuclear explosions in general.
Several plutonium bombs were now "in the pipeline" and would be available over the next few weeks and months. So it was decided to test one of these.