William and Harry, the second and fifth in line to the British throne, were 15 and 12 respectively when Diana, Princess of Wales, died.
They have opened up about their mother who was killed in a car crash in Paris on August 31, 1997, for this first time in the BBC film tentatively titled "Diana".
"Part of the reason why Harry and I want to do this is because we feel we owe it to her. I think an element of it is feeling like we let her down when we were younger. We couldn't protect her," says William, the Duke of Cambridge, in the documentary.
His younger brother Prince Harry said the outpouring of emotion and love following the death of Diana was "shocking" to him as a young boy.
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"It was beautiful at the same time, and it was amazing, now looking back at it, it was amazing that our mother had such a huge effect on so many people.
"I think it's never going to be easy for the two of us to talk about our mother, but 20 years on seems like a good time to remind people of the difference that she made not just to the royal family but also to the world," he said.
It will also feature interviews with close friends, political figures and journalists, some of whom are speaking about Diana's death and its aftermath for the first time.
It will also air a new drama, "Diana and I", in which the impact of Diana's death on the lives of ordinary people is explored.
Alison Kirkham, controller of factual commissioning at BBC said: "We are living in a period of seismic change when it feels harder than ever to get to grips with what is happening around us.