Simple question... Deep Purple.

"Speed King" is the first Power Metal song that was ever conceived and recorded!!!

And "Fireball" was the first Prog Metal album that was ever conceived and recorded!!!

 

They did have the volume, I'll give them that... legend has it that they turned the air around them into something like lithium-flavoured candyfloss, and could be heard from 10 miles away - not sure if they ever made it to 11...  

 

It's a common fallacy that Steppenwolf were the originators of the term;

 

It was either an unconfirmed journalist of 1967 who described Hendrix's music as "Heavy metal falling from the skies" (or something like that - like I say, it's unconfirmed, but I've heard this rumour many times, and there's no smoke without fire)

 

- or it was the album Hapshash and the Coloured Coat, featuring the Human Host and the Heavy Metal Kids, released in 1967. The music isn't metal - not even close - but the band previously gave Hendrix his debut gig in the UK, played music that was undoubtedly heavy for the time, and went on to become a major root of metal music, influencing Sabbath, Purple, Zep, Blue Oyster Cult and Judas Priest.

 

I'd support Mountain. Cream - and probably The Doors and The Who - in proto - but would be a bit wary of too many hard rock acts in there; Bands like Free, Fuzzy Duck, Leafhound, Bakerloo, Quatermass, May Blitz, Sir Lord Baltimore et al are bound to come up for scrutiny, and while the "heavy" is indisputable, the "metal" is highly questionable.