Code-named “Fat Man” for its roundish shape, this was the experimental bomb that used Plutonium rather than Uranium. They weren’t 100% sure it would work so they actually tested the first atomic bomb, using Plutonium, a few weeks before they dropped its twin brother on Japanese.
It is said that while the Plutonium bomb had a greater yield than the Uranium bomb dropped over Hiroshima, the destruction was more limited – with an elongated area of complete destruction because Nagasaki is surrounded on 3 sides by hills. So while the zone of utter destruction is only 2 km wide in the east-west direction, it is about 6 km long in the north-south direction.
And I still find it curious and odd that the Manhattan Project, inspired by Albert Einstein, a reputed peacenik, who was concerned with German atomic scientists getting a bomb built before America had one, ended up producing 3 bombs, testing one, and dropping the other two not on Germany which never even got close to producing an atomic bomb, but on Japan.
It also sort of makes me wonder what we would have done if the Japanese knew that was all the bombs we had, and that our threat to bomb their islands to a dust heap was a complete bluff.