I'm sure this is a source of raging debates. I think Black Sabbath beat them to producing something that could be called heavy metal. Just doing a little googling, I see The Beatles, The Who and Led Zeppelin also mentioned. I've also seen it argued that certain classical music pieces were pretty much "metal," they just didn't have electric guitars yet. Fun topic.
George Harrison claimed that Ticket to Ride was the first heavy-metal song because of the 12-string Rickenbacher he played on it. I think if that can constitute heavy metal, then I'd give the Kinks the honor, with Dave Davies being the first to play a guitar with heavy distortion (achieved by taking a pen knife to a speaker).
I read somewhere that the term heavy metal was first used (wrt music) to describe the Jimi Hendrix Experience. I don't generally think of Hendrix as metal, but that's much less of a stretch than the Beatles. I have head-banging friends who consider Zeppelin metal, but to me they were so blues oriented that they're firmly in the rock camp. There's no question about Sabbath, though, who predate Smoke on the Water by a few years.