In an interview with the Forgotten Hits newsletter/website, Bill Medley stated that Phil had left a recording session 15 minutes early and told them they could use the remaining time however they wanted, so he (Bill) produced Unchained Melody, and Phil stuck it on the back of Hung On You as a throwaway B side without mentioning any producer credits on that side (again, if Phil had produced it, even as a throwaway B side, his name would've still been on the labels).
As far as I'm concerned, it sounds like a Bill Medley production and he claims he was the producer of it. I doubt it had any Spector involvement at all, and as such, do believe Spector should not be given producer credit on that side.
Even on throwaway B side instrumentals that had no chance at being played on radio, Phil had his name listed as producer on the track. If Phil had produced Unchained Melody, I have no doubt his name would've been on the label from the very beginning.
The deal with Phil Spector and the Righteous Bros. was that Phil could produce whatever
records he chose. He chose "Hung on you" as their 2nd big one.
On the back he put "Unchained Melody" as a "co" production without any mention of producer.
His effort was to add the strings,some drums and background vocals and the remainder be
a joint effort. As soon as DJs flipped it over he had new labels printed.
‘Unchained Melody’ is unusual in that all the singing on the disc is done by Bobby Hatfield. As far as being produced by Bill Medley goes, it's possible that Bill Medley might not even have been in the studio.
When the 45 was flipped over by DJ's and Unchained Melody became the more popular side, the 45 was actually re-issued with Unchained Melody as the A-side, adding the Phil Spector credit to the label. This does not imply it was or was not produced by Phil. It merely casts doubt upon it.
Notice DJ copy "B side" doesn't say "Produced by Phil Spector"..theories suggest that Unchained Melody was actually produced by Bill Medley, when DJ's flipped the single and started to play the "not-produced by Phil Spector side". Phil immediately had the next pressings add his production credit ...of course.