Former plane navigator Jack Lyon was number 79 on the list of PoWs preparing to break out of Stalag Luft III in 1944.
He was about to enter one of three escape tunnels the prisoners had dug when a gunshot was heard and they realised the game was up.
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A total of 76 men had earlier gone down the tunnel but of those just three made it to freedom.
The rest were recaptured and 50 were executed on the direct orders of a furious Hitler.
The story was told in fictional form in the classic 1963 film The Great Escape starring Steve McQueen and Richard Attenborough.
But for Jack, who opened his 100th birthday card from the Queen at a party organised by the RAF this month, it was for real and could have seen his life cut short.
He said: “Another half an hour and I wouldn't have been here now.
Air Commodore Charles Clarke recounts Great Escape 1944
Another half an hour and I wouldn't have been here now
“When I think about it, luck was on my side. Let’s face it – the chances of making it out even in better conditions were slim.”
Jack was a flight lieutenant when his bomber crashed in Poland and he was captured.
Three years later he joined the Great Escape team, acting as a look-out on the night of the break-out.
He recalled: “Once the game was up everything incriminating was burnt.”
But he added: “Everybody retained a certain attitude of optimism. The odds were always against it succeeding.”
Widower Jack now lives on his own in Bexhill, East Sussex, and travels around the country giving talks about his experiences.
He said that, unlike in the film, no Americans were involved.