They had already bombed Tokyo into ruins five months prior, killing at least 100,000 people and leaving a million homeless. It was the most violent air raid in history, doing more damage than either of the atomic bombings.

The novelty of the atomic bombing was not that the USAF could destroy a city, because they'd been doing that over and over for a year already, but that they could now do it with a single plane and a single bomb. They specifically selected a city which had never been raided before so it would be clear that all the damage had been done by the new weapon.