At http://www.bbc.co.uk/history/ww2peopleswar/timeline/factfiles/nonflash/a6652262.shtml it is written:
Hiroshima was chosen because it had not been targeted during the US Air Force's conventional bombing raids on Japan, and was therefore regarded as being a suitable place to test the effects of an atomic bomb. It was also an important military base.
Was it spared conventional bombing purely so it could be hit with an atomic bomb? Or was it spared conventional bombing for some other reason?
Hiroshima, Kokura and Niigata were cities that, in the process of atomic bomb targeting, had been added to a list of "reserved cities" specifically so that the effects of the bomb(s) could be studied. Our resident nuclear expert, /u/restricteddata, has an article posted on the New Yorker's website today that talks about this, among other things:
http://www.newyorker.com/tech/elements/nagasaki-the-last-bomb
Also, if you're interested in the atomic bombings or Manhattan Project, you should check out our section on them in the FAQ and also /u/restricteddata's recent AMA. Links below:
https://www.reddit.com/r/AskHistorians/wiki/wwii#wiki_the_atomic_bombs
https://www.reddit.com/r/AskHistorians/comments/3e76ig/ama_the_manhattan_project/