The most important question in the history of heavy metal - to some, at least - is the question of origin: what was the first heavy metal band, album or song? I'm gonna take my personal shot at answering this very complex issue, so, for what it's worth, this is my attempt at an explanation. But before we dive into the question, I want to take a small amount of space in the infinity of Internet to point out, yet again, that this is just one, very subjective opinion and there are others, most as equally valid as mine.
The thing with the first "heavy metal band" is that most of the proto metal bands aren't exclusively... how'd ya call it... metal. Yea, sure, Led Zeppelin did write some very metallic anthems, full of groove, loaded with killer riffs etc., but they also made a lot of stuff that is completely removed from the sphere of heavy metal, and same goes to Deep Purple and even the highest authority in anything metal, Black Sabbath. To me, the answer to the question of origin must be a band and record that is both timeless and unique, but not too much, since it/they would have had spawned a whole genre of music, legions of imitators that sound themselves and also the object of their imitation.
I personally find the quest for the first metal song to be fruitless and totally pointless search of a needle in a haystack. Some will go on a record saying that all metal begun with Steppenwolf's Born to Be Wild, since the song mentions "heavy metal thunder"; some will argue about the superiority of The Beatles' Helter Skelter; some are willing to go back to the classical roots, in the masters like Bach and Beethoven. Heaviness is an element in all of these, no doubt, but a whole genre of music cannot, in my opinion, come into being from just one song or one composition, no matter how "heavy" or "metallic" it may be - and there is no point in saying that The Beatles did make a lot more than Helter Skelter, Steppenwolf barely anything else than Born to Be Wild and Bach/Beethoven are from a completely different cultural sphere and time than the music we now call heavy metal. The beginning of heavy metal cannot be found in neither of these glimpses from the misty haze of prehistory of the mind-shattering distortion.
There are three big bands from the UK that usually get the honor of being the first heavy metal band and/or having made the first heavy metal record, depending who's giving the statement: Black Sabbath (most notably Black Sabbath and Paranoid are on the two top steps of the podium when contesting for the first heavy metal record), Deep Purple and Led Zeppelin. I'm going to take a rather different stand and say that the first truly metal band is none of the above, although the title of first record is a bit more difficult to determine. What I'm going to do next is a sort of a dissection of the myth that these three bands created heavy metal as a music genre, but you have to remember, that all the bands listed above were prime innovators in proto metal and had a huge impact on metal.
Most common answer to the question of origin seems to be Black Sabbath, and for a very good reason: the riffs of Tony Iommi, the drumming of Bill Ward, the occultistic lyrics of Geezer Butler and, of course, the vocals John Michael "Ozzy" Osborne captured the minds and ears of the hard rocking audience in early '70's, and even today, nearly 50 years after Black Sabbath came into being, what these four blokes from Birmingham created, holds a sanctified position in the minds of metalhearts around the world. I'm not going to lie: I personally rallied years for giving Black Sabbath the honor of being the first heavy metal band, but recently my point of view has shifted away. The main point why I no longer consider Black Sabbath the first heavy metal band is that they were too unique. This might sound very weird and dumb, but the point being that if a new band imitates Black Sabbath, they don't sound unquestionably "heavy metal" but instead "Black Sabbath ripoff", and therefore a whole genre of music cannot have been spawned from Black Sabbath. If a band X wants to go to roots of heavy metal, the first thing they pick up is some early Black Sabbath records, but usually drift away from this, since they can never truly sound solely heavy metal, but instead Black Sabbath ripoff, and when they realize this, they usually move to another band, which will be mentioned later on.
Did you get my point? Don't worry, it's just the whole skeleton of my text. I'm gonna refer back to this - half-assed? fumbling? dumb beyond belief? - argument from time to time and hopefully it will be clearer by the end.
Deep Purple is the easiest to rule out, since they were always musically moving around: In Rock was very straight-forward hard rocking record, Fireball lazily put together mess, Machine Head brilliant evidence in showmanship, Burn and Stormbringer had much more funky sound etc. They flashed on the metaldetector from time to time - most notably Fireball, Speed King and Burn - but they never made unquestionable "heavy metal" record and every one of 'em branched out to other genres. To me, this rules Deep Purple out from the competition - even though, as I said, they (Richie Blackmore inspired generations of guitarist, for one) influenced the birth of metal.
Led Zeppelin was always a sort of a watershed. I've never been a big fan - yeah, sure Stairway to Heaven is a classic and Rock And Roll rocks and rolls my freaking brains out - and I've felt kinda awkward when people start arguing for the favor of Jimmy Page and buddies. A question that pops in my mind when I hear people say this is: "Have you actually heard IV? It's not metal, it's hippy-bullshit!" Again, I've never been a fan, so rule this statement out of your mind, if that's what you want, but consider this while you do it: Misty Mountain Hop, Battle of Evermore, Going to California, When the Leeve Breaks - are these a staple of a heavy metal record? One or two of these on a "pure" heavy metal record might be acceptable, but half the album? Really, honestly? Sure, keep your bubble, mate, but please don't get mad for me pricking it with a needle.
So, if none these massively influential bands weren't the "original heavy metal band", what was/is? What is the record that has been imitated from there on - a LP-long piece of music, which is undoubtedly metal and only metal, not hippy-bullshit, not funk, not blues with bigger riffs? And keep in mind it has to be a big band, not a minor group that made one single in the early '60's.
I'm sure you know the band and album I have in mind, so I'm not gonna drag this on any longer: Judas Priest and Sad Wings of Destiny. Yeah, obviously. Like the "big three", from the industrial cities of UK, Judas Priest's second record (Rocka Rolla just isn't good enough, honestly) is hard rocking record with exceptional depth. Even today it sounds fresh, grooving and angry - yeah, you can hear it's age here and there, but I think it has survived much better than IV, for example, which to me always sounded too much like hippies' drumcircle-jamming. The Ripper is a classic anthem, Victim of Changes still a staple of Judas Priest's liveshows and not to forget very pissed off Tyrant. K.K. Downing's and Glenn Tipton's riffing and dual guitar solos are heard all across the board in metal - from thrash giants (like Slayer, Metallica and the rest), through power metal (Gamma Ray, most notably) to the rest of the genre, in one form or the other. Who heavy metal vocalist haven't, at least once, tried to be Rob Halford, soaring miles higher than guitars, screeching like a banshee getting kicked in the groin? This is heavy metal, this is the root of everything metal - from Victims of Changes to Isle of Domination and not to forget that amazing cover art. No Judas Priest, no NWOBHM - no NWBHM, no speed/thrash metal etc. This is the point of origin.
There we are, the question of origin has yet another answer. I know Sam Dunn made the same point - that Judas Priest is the first real heavy metal band - but his argument was that of bands rhetoric of being labeled heavy metal band, which is something Black Sabbath, Deep Purple and Led Zeppelin always shunned. For what it's worth, this is my argument: Judas Priest is the first heavy metal band, since they were the first to make a unique but not too unique (hard) rock masterpiece, that became the groundwork for the whole genre of music which nowadays has adapted the name of heavy metal.
P.S. I'm awfully sorry this has taken a long time to write. I have only excuses.