Those who were there still recall how dazzling Diana was that night. She wore diamond and pearl drop earrings and a velvet gown that was alluringly low-cut.
Some 900 of New York's most powerful movers and shakers had gathered at the Hilton Hotel in Manhattan for a glittering $1,000-a-plate charity dinner. It was December 1995, and fate had placed Donald Trump on the princess's table, although his second wife Marla was also present.
Diana had greeted him and other major donors earlier at the reception. She'd already met the tycoon several times, mostly at charity functions, but didn't know him well, although he'd offered her complimentary membership of his Florida country club, Mar-a-lago - an offer she declined.
She was certainly well aware of his reputation with women, though he had never made a pass at her. For his part, Trump had noted Diana's qualities in some detail.
As he wrote in his book, The Art Of The Comeback, published in November 1997, three months after her death: 'I couldn't help but notice how she moved people. She lit up the room. Her charm. Her presence. She was a genuine princess - a dream lady.'
Totally unaware of how he felt, you can imagine Diana's surprise soon after she and Charles were divorced in July 1996 - six months after that Manhattan dinner - to receive a huge basket of flowers from Trump.
They were for her 35th birthday. Clearly the lascivious billionaire was making the princess an exception to his locker-room maxim that 'it's checkout time' for women once they reach the age of 35. (Of course, history then could have taken a very different course. If they had hit it off, she might now be about to move into the White House as America's First Lady - never having dated Dodi Fayed with such tragic consequences.)
This week, however, it emerged how, two months after Diana was killed in that car crash in Paris, Trump talked about her with rather less respect than he showed in the book.
Rather, she was a potential sexual target.
At the time, he was a guest on an American radio show presented by 'shock jock' DJ Howard Stern and he boasted, on air, that he 'could and would' have slept with Diana.
'You could have nailed her,' suggested the DJ crudely, to which Trump replied: 'I think I could have . . . She had the height, she had the beauty, she had the skin.'
Nothing, notice, about what it was that he thought might make him attractive to her.
For these were the obnoxious thoughts of a rich man whose own marriage was beginning to crumble and who believed his charm, fortified by his money, could buy him just about anything, or anyone. Two years later, his marriage to Marla was over after just six years.
The truth is Trump hadn't said much to Diana that evening in New York. In fact, the one person Diana was really hoping to meet didn't show up.
This was John F. Kennedy Jnr, son of the assassinated JFK and, at 35, a year older than Diana and said to be the world's most eligible bachelor.
Young JFK Jnr was told later that he 'missed a treat' - Diana was so bewitching that she was described as having 'won America'.
As for Trump, his remarks at the Hilton gala were confined to expressing his admiration for the way Diana handled a woman heckler who interrupted her speech.
It happened when the princess was talking about people in need, especially referencing parents with small children, when a middle-aged woman shouted out: 'Where are your children, Diana?'
'At school!' shot back the princess, and she moved seamlessly on.
Trump, then aged 49, was seen nodding his approval at the way Diana had handled the tricky situation. He was impressed, adding somewhat threateningly about the heckler: 'I would have liked to have had a word with that woman.'
No doubt he would have liked to have had more than a few words with the princess, who that night was sitting between two of the most respected figures in America, the celebrated veteran statesman Henry Kissinger and Colin Powell, the U.S. Army four-star general who later became Secretary of State under President George W. Bush.
The vain Trump clearly assumed he could be in with a chance. After all, the princess had then been separated from the Prince of Wales for more than three years and had been enjoying the company of another billionaire American.
He was Theodore 'Teddy' Forstmann, private equity pioneer, philanthropist and legendary ladies' man who was also one of the richest men in America. She spent time with the bachelor - skiing with him in Colorado, playing tennis at Martha's Vineyard, the exclusive island off Massachusetts, and borrowing his ultra-luxury Gulfstream V private jet (Forstmann owned Gulfstream) to fly from New York to Washington.
How Trump envied Forstmann. Not just because he was many times richer than him, but because, when talking casually of his female conquests, he always included Princess Diana in a list that also boasted actress Elizabeth Hurley and novelist Salman Rushdie's glamorous former wife Padma Lakshmi.
Diana had been introduced to music-loving Forstmann - a pianissimo personality to Trump's fortissimo - in June 1994 by grandee banker Lord Rothschild. They were at Spencer House, her family's ancestral home in London, and the occasion was a charity dinner (Diana was always meeting billionaires at charity dinners).
So taken with her was the portly, 53-year-old tycoon, that the following day a huge bouquet of mixed flowers arrived at her apartment in Kensington Palace. They continued to arrive every week for the next three years, as they remained friends.
Looking back at these times, it is ironic to reflect - and perhaps Donald Trump already has - that during this period, Forstmann was being talked up in political circles as a possible Republican candidate for the 1996 U.S. election (won by Democrat Bill Clinton for his second term).
So could Diana's friendship with Forstmann really have evolved into something serious?
With a straight face, she would say to close friends: 'How do you follow being married to the Prince of Wales unless your next husband is President of the United States?'
Deep down, however, Diana's mood was complicated.
She mused on the way she had been feted that night in New York and how different this was to her mistreatment by the court back in London.
It was only a month earlier that she had outraged the royals by giving that extraordinary interview on the BBC's flagship Panorama programme, in which, watched by the nation, she not only questioned Prince Charles's suitability to be king, but admitted having been in love with Cavalry officer James Hewitt.
Within days of returning from that dinner, she had been brought crashing to earth when she learned that the Queen was instructing her and Charles to end the uncertainty about their futures and to divorce.
So just imagine the public outcry if, out of all this, she had emerged on the arm of one of the U.S.'s brashest billionaires.
For his part, Trump was years away from following Forstmann in turning his mind to thoughts of occupying the Oval Office when he dispatched those first flowers - roses and orchids - to Princess Diana after her divorce.
They were sent in the wake of what he has called a 'favour' he did for the princess, one he has not explained further. We understand it was a special donation to one of her favourite charities. Typically, Diana sent him a hand-written letter, thanking him in warm terms.
For the narcissistic and socially conscious Trump, receiving such a personal letter from the Princess of Wales was a coup. The huge bouquet was his response.
What he didn't know was that Forstmann was still sending his weekly bouquets, which continued arriving until Diana's death. Trump attached a message expressing his sympathy over her divorce, his regard for her and even, it is said, suggesting they get together. According to broadcaster Selina Scott, more flowers followed.
As she told the Sunday Times: 'He bombarded Diana at Kensington with massive bouquets, each worth hundreds of pounds. Trump clearly saw Diana as the ultimate trophy wife. As the roses and orchids piled up at her apartment, she became increasingly concerned about what she should do. It had begun to feel as if Trump was stalking her.'
But Diana did not view Trump in the same way she viewed the softer Teddy Forstmann.
Over dinner, she apparently told Selina Scott: 'He [Trump] gives me the creeps.'
And then his flowers stopped. Having had no encouragement, Trump, ever the businessman, cut his losses. It simply became apparent that he was wasting his time.
If only he had known what was really going on in Diana's life, he would have realised that the last thing on earth she needed at that time was another dubious suitor. In fact, both Forstmann and Trump were wasting their time.
For Diana had met someone else, someone with no private jet, little money, not even his own home. He was a junior heart and lung surgeon at London's Brompton Hospital.
Born in Pakistan, his name, of course, was Hasnat Khan. They met when he operated on the husband of one of Diana's confidantes, healer and former nun Oonagh Toffolo.
This man - of whom she said: 'He's drop-dead gorgeous' - was the heartfelt answer to Diana's anguished question about how she was supposed to 'follow being married to the Prince of Wales'.
She would, as the world was to learn, be most happy to live with Khan - and even move with him to Pakistan if need be. In the end, it was he who turned her down, saying it would be wrong for her.
It remains, however, an intriguing thought that if Donald Trump had won her over, she really might have stepped from being the wife of the Prince of Wales to being the wife of the President of the United States.
These days, Hasnat Khan is a consultant heart surgeon, saving lives all over the world. John Kennedy Jnr was only 38 when he was killed, along with his wife, in a plane crash in 1999. Teddy Forstmann died in 2011, still a bachelor, aged 71.
And Donald Trump prepares to enter the White House with wife No. 3 in January.