What a lot of the world didn't know was that the fancy wedding dress she wore wasn't her only dress option. There was, on reserve, a backup wedding dress which would act as a stand-in if Diana's dress was revealed before the big day.
Now, it is just being shared for the first time.
"At the time we wanted to make absolutely sure that the dress was a surprise," said Elizabeth Emanuel of Elizabeth and David Emanuel designs, who created both gowns. "Had the secret of the real dress got out it's possible that Diana would actually have worn this one."
The ivory silk taffeta gown, bearing the same ruffles around the neck as the original but without the signature lace, was created in case of emergency, but was never completely finished.
"We didn't try it on Diana. We never even discussed it," said Emanuel. "We wanted to make sure that we had something there; it was for our own peace of mind, really."
Emmanuel adds now that she can sympathize with whomever is designing Kate Middleton's dress for her big day with Prince William.
"They're keeping quiet on the designer this time, probably to avoid all the hassle that we had," she said. "It was really difficult and we didn't have to contend with cameras in phones etc like you have today. How do you keep something secret like that? I have no idea."