As I understand it the Uranium bomb design had been tested beforehand but it was not necessary to actually explode a device in order to do so.
The bomb achieved critical mass by firing a cylindrical “bullet” of uranium down a converted gun barrel into a circular uranium target – by slowly sliding the bullet down the barrel and measuring the increase in the flow of neutrons as it approached the target.
They were able to go to something over 99% of critical mass and measure precisely how and when the bomb would explode without actually setting the bomb off so they could be sure that it would work when dropped operationally.
With the Plutonium design there were technical reasons why the “gun” method could not be used and an implosion design was the only viable alternative - that could not be tested other than by detonating a live weapon.