Ironic, then, that The Martian Chronicles was the book that first branded him with the science fiction label.
Bradbury's views on his own writing were being clearly expressed way back in the 1950s, and probably earlier. In 1951, the year after The Martian Chronicles was published, he was interviewed by Harvey Breit for the New York Times.
Breit systematically asks Bradbury for his definitions of SF and fantasy. Bradbury makes his distinctions clear, with this description of science fiction:
Science fiction is really sociological studies of the future, things that the writer believes are going to happen by putting two and two together [...] Science fiction is a logical or mathematical projection of the future.And as for fantasy?
It's the improbable. Oh, if you had a leprechaun or a dinosaur appearing in the streets of New York - that's highly improbable.In light of these definitions, it is quite clear why Bradbury continues to characterise Chronicles as fantasy and 451 as science fiction. It is evident that Bradbury's take on SF is that it is primarily about warning us about the future ("If This Goes On...", to borrow a short story title from Robert Heinlein.
Of course, 1951 was also the year of publication of The Illustrated Man, a collection of mostly SF short stories, most of which take the line of "if this goes on". In the Times interview, Bradbury says, "The mechanical age is crushing people. People are confused", and this attitude is reflected in various ways in some of the stories in The Illustrated Man and in Fahrenheit 451. It is this warning about the future that earned Bradbury another label he has struggled to shake off: as someone who is anti-science and anti-technology.
Interestingly, the Times interview ends with Bradbury showing his optimistic side: "When we move out into space, what a revolution!" he declares.
(Source: "Talk with Mr Bradbury", New York Times, 5 Aug 1951, p182)