Who invented heavy metal?

The phrase heavy metal (in music) was literally invented by Steppenwolf - Born to be Wild (1968).

For opinion on the question, read on.

The Kinks' You Really Got Me (1964) was probably the first example of a metal riff (where Davies reportedly slashed his amp with a knife to get distortion), and Hendrix pioneered the guitar histrionics that characterise a lot of metal. Most people say that 'metal' itself was created by Black Sabbath (1968) though; riffs that were no longer so bluesy, the occult theme, the heaviness, the power chords, the pessimistic outlook. It's possible to trace most aspects of metal back to Black Sabbath.

Other bands such as Blue Cheer (1967) and High Tide (1969) are also credited with inventing metal.

I'd also like to shout out Gustav Holst (1916) and Jeff Beck, who is credited with being the first guitarist to create a way to control guitar distortion (circa 1966) and, (just my opinion) recorded the first metal-style guitar solo on Heart Full of Soul with the Yardbirds, released in June 1965.