A trailer for Diana, Our Mother: Her Life and Legacy was released today and shows both of the princes leafing through a family album of photographs.
The documentary, which is due to air on ITV in Britain and on HBO in the US later this month, will explore the relationship between Princess Diana and her two sons.
Speaking in the film, which will also air on CBC in Canada, Princess Harry said: 'This is the first time that the two of us have ever spoken about her as a mother.'
Diana, Our Mother: Her Life and Legacy, coming soon to ITV
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He added: 'She was one of the naughtiest parents. She was our mum. She still is our mum.
'And of course, as a son I would say this, she was the best mum in the world. She smothered us with love, that's for sure.'
Studying a photo of Diane holding him whilst pregnant with Prince Harry, Prince William says: 'Believe it or not, you and I are both in this photograph, you're in the tummy.'
In April, Prince Harry revealed how William encouraged him to seek counselling after two years of 'total chaos' as he struggled to come to terms with his mother's death.
The 32-year-old said Prince William implored his younger sibling to get help after 'shutting down all his emotions' for nearly 20 years following Diana's death in 1997.
But the prince, who was 12 years old when his mother died, admits that he ignored his grief during his teenage years, only addressing it when he was 'on the verge of punching someone' in later life.
Before that point, he endured two years of 'total chaos' when he became 'a problem' to himself - a period which included the infamous Nazi fancy dress and intimate photographs from a party in Las Vegas - but 'did not know what was wrong'.
But the royal - who even turned to boxing to help ease his aggression - says he was saved by opening up about his feelings, leaving him 'in a good place' and wanting to do all he can to help remove the stigma around mental health.
Prince Harry was only 12 years old when Princess Diana died in a car crash in on August 31, 1997.
Diana was killed when the car carrying herself and her lover Dodi Fayed smashed into a pillar inside a Parisian tunnel in the early hours.
The vehicle was being driven by the pair's drunk chauffeur Henri Paul, who attempted to outrun the pursuing paparazzi.
Their deaths would send shockwaves around the world and triggered an unprecedented outpouring of grief in Britain.
ITV's Jo Clinton-Davis said: 'This new ITV film will offer viewers a fresh and revealing insight into Princess Diana through the personal and intimate reflections of her two sons and of her friends and family, many of whom have never spoken before, to bring together a definitive portrait of a unique person who touched the lives of millions.'
Executive producer Nick Kent added: 'This film will show Princess Diana in a way she has never been seen before, through the eyes of the two people who knew her best.'