Hiroshima, (with Nagasaki or Kokura as backup plans) were the chosen as targets for bombing because they were larger than three miles in diameter, the blast would do enough damage there, and because they were very populated and had military bases.
The first weapon was to be dropped on Hiroshima (on August 6th). An hour before it happened, japanese radars detected American aircrafts coming into southern Japan. They broadcasted it by radio and other media. But then they figured out there was only about three small planes and lifted the big warning. Still though, they said to have people go to bombing shelters if any B29 planes were seen. The bomb was dropped at 8:15 and took only 43 seconds to hit ground. Americans estimated that about 4.7 square miles of the city were totally destroyed. 70,000-80,000 people in Hiroshima died immediately, and about 70,000 were injured.