Let's start with the fact that it was not written by, or for, the Righteous Brothers - it was already ten years old before they released it as the B-side of the single Hung on You, penned by Gerry Goffin and Carole King. But DJ's preferred the B-side, and played that instead.
This rebellion of the radio stations (who were being bribed to play what he wanted them to play, after all) annoyed producer Phil Spector; one continuing rumor says that the main reason was that Spector hadn't actually produced Unchained Melody - it was just a B-side, so he'd left those duties to Bill Medley, one of the two singers (which is why, if you listen, it's essentially a solo by Bobby Hatfield, the other half of the duo - in fact, Hatfield later recorded it as a solo, after the duo broke up).
The Righteous Brothers' 1965 original became a hit again when it was used as the love theme for the 1990 movie Ghost.
(This video uses Italian overdubs of the dialog, because Paramount cracks down on clean copies of the original whenever they're posted on YouTube.)
Unfortunately for the movie studio, they underestimated how big a hit this 25-year-old re-release was going to be, and didn't use it extensively to promote the movie. It was only available as a 45 RPM single, which sold out quickly. The Righteous Brothers, who weren't making a dime off the song anymore, quickly rerecorded it, and Curb Records released it as a cassette single.
The original 1965 version, with limited sales and on the strength of the radio airplay, reached #13 on the Top 40 charts. The rerecording, with limited airplay and on the strength of sales almost entirely, reached #19. And for eight weeks in mid-1990, both songs were simultaneously sharing space in the Hot 100; the re-release became their second UK #1.
Despite being the most famous version, the Righteous Brothers' versions were not the first. The song was originally written as the theme for a 1955 prison movie called "Unchained."
This explains the title of the song, since the lyrics don't actually contain the word "unchained" at all. In fact, it brings a new light on lyrics which I've always heard but never thought about.
Oh, my loveThe song was written by Alex North and lyricist Hy Zaret, and operatic baritone Todd Duncan sang it for the movie. As well as the soundtrack version, six different artists (Les Baxter, Al Hibbler, Jimmy Young, Roy Hamilton, Liberace and June Valli) released versions in 1955 which charted in both the US and UK (Valli's version being the only one that wasn't a Top 10 hit in one country or the other).
I've hungered for your touch
a long lonely time
and time goes by so slowly
and time can do so much
are you still mine?
...I'll be coming home,
wait for me
The next year, it was nominated for the Academy Award for Best Original Song of 1955; at the ceremony, it was sung by Harry Belafonte, who had just released his own version of the song.
It has gone on to become one of the most recorded songs in history, released by over 500 artists in dozens of languages. It's been done by Roy Orbison, U2, the Supremes, Sarah McLachlan, Ricky Nelson and Cyndi Lauper, among so many others.
In his last television appearance in June 1977 (six weeks before his death), Elvis Presley performed the song for Elvis in Concert. It was also on Moody Blue, the last album released while he was alive, with Elvis accompanying himself on piano.
There was also one other version performed in 1955, though. It was from an album by "the Goons," composed of Peter Sellers, Spike Milligan, Harry Secombe and others. But the publishers of the song didn't appreciate the flippant version, and it remained unreleased until 1990.
(Note: h/t to Left-Leaning Lady for catching my typo on Elvis' performance. Date corrected.)