Originally Posted by
altaskier
At the time it was much harder to do isotopic separation (U235) than chemical separation (Pu238 made in reactors). At the same time, plutionium requires implosion detonation which is rather tricky while uranium can work in a simple gun-type bomb. Therefore it was felt that there was no need to test a uranium gun-type bomb, but there was a desire to test a plutonium gun-type bomb rather than have it go fizzle in a bombing meant to have frightful impact on an enemy...
Incidentally, there is no such thing as a plutonium gun-type bomb. It won't work--you'll get a fizzle every time. Gun assembly is simply to slow.
(Note that the reverse does not apply--you can make a U-235 implosion bomb.)