The analogy is not quite fair. Little Boy’s forerunner was tested.
Fat Man was hardly the forerunner. Fat Man was the more elegant, but harder to implement, design. That's why it was tested.
Little Boy was the fallback in case they couldn't solve the complex engineering problem that was Fat Man's implosion design. IOW, they knew the implosion method was the right way, but they also knew the crude gun design would work and impress the bejesus out of the Emperor. And they had Little Boy ready to go, so they dropped him, and he worked!