I'd go back a year or two and give a lot of credit to Jimi Hendrix. He pretty much took Buddy Holly's set up and used analog or transistor signal processing like the wah-wah, phase shifter, and distortion while throwing in a whammy bar, overdriving tube amps, and speed and created a heavier version of Rock and Roll. Listen to That'll be the Day by BH and then Machine Gun by Hendrix (especially about halfway in). Drums, bass, electric guitars plugged into tube amps. Listen to what Jimi did with Bob Dylan's folk songs (All Along the Watchtower, Like a Rolling Stone).
Deep Purple along with Zep and Sabbath were pioneers in the sound as well. They have songs and/or portions of songs that are heavy metal although their music is far more diverse. Even the Beatles dabbled around with it (Revolution). John Lennon especially turned up the dirty distortion (listen to the 18 bars of (1, 4, 7) McCartney to (2, 5, 8) Harrison to (3, 6, 9) Lennon trading licks in The End on side 2 of Abbey Road).
Heavy Metal "bands" focus on that element of Rock. Kind of deemphasizing the blues roots, exclude acoustic compositions and ballads, and keep the tempo fast and the amplification dirty. But even heavy metal bands will throw in some ballads. Judas Priest's Beyond the Realms of Death for example. They're all just branches coming from the same tree. Rhythm and Blues. Rock and Roll. Rock. Hard Rock. Heavy Metal. Acid Rock. Punk Rock. Thrash Metal. Speed Metal.