In a documentary called Diana, Our Mother: Her Life and Legacy timed to coincide with the 20th anniversary of Diana's death in a Paris car crash on Aug. 31, 1997, the princes said they spoke to their mother shortly before she died.
"Harry and I were in a desperate rush to say goodbye, you know, 'See you later' ... If I'd known now, obviously, what was going to happen, I wouldn't have been so blasé about it and everything else," Prince William said.
His younger brother Harry said he remembers Diana calling from Paris but little else about the conversation.
"I can't really necessarily remember what I said but all I do remember is probably regretting for the rest of my life how short the phone call was," he said.
Nick Kent, the film's executive producer, told Reuters it offers a glimpse of "the private Diana."
"Nobody has ever told this story from the point of view of the two people who knew her better than anyone else, and loved her the most: her sons," Kent said.
The princes recall their mother's sense of humour, with Prince Harry describing her as "one of the naughtiest parents."
A film for the royal grandchildren
They also recall the pain of their parents' divorce and how they dealt with the news of her death and its aftermath.
While the film addresses aspects of Diana's life such as her charity work involving HIV and land mines, it shies away from her and Charles's rumoured extramarital affairs.
According to the film's creators, the Royals were very open and did not make any subject off limits. Kent said they were eager to cover new ground and make a different type of film.
"What we had in mind is that in years to come, Prince William and Prince Harry would be happy to show this film to their own children and say, 'This is who your grandmother was,'" Kent said.
Several events to mark anniversary of death
A number of commemorative events have been planned to mark the 20th anniversary of Diana's death. William and Harry attended a private service this month to rededicate her grave, and the brothers have commissioned a statue to be erected in her honour outside Kensington Palace in London.
Rarely seen possessions of Diana, including her music collection and ballet shoes, went on display on Saturday at Buckingham Palace. An exhibition celebrating Diana's fashion opened in February.