[Timesink]

As I recall, they never even tested Little Boy. Dropping it on Hiroshima was the test; that's how certain they were that it would work.

[Tijeras_Slim]

That is correct. The Little Boy device was simple in a physics sense, but difficult in a materials sense (Y-12 in Oak Ridge which was, I believe, the largest roofed structure in the world at its time, was built to enrich uranium by just one method) There were several methods used simultaneously, and it required tons of U238 to produce small quantities of U235. Coupled with this, the gun-design was inefficient, yielding (I think) about 15% of its potential. The High Energy Weapons Archive website is an excellent resource, with a minimum of bias.

[TiaS]

They did test the atomic bomb at Trinity Site in southern New Mexico a month before they dropped the bomb on Hiroshima. At that time, they knew little about what would happen there, Edward Teller has been quoted as saying he believed it was even possible the explosion would be so powerful it could ignite the earth's atmosphere -- they did it anyway.