McQueen was speaking before its European premiere as part of the BFI London film festival while at the same time it was going on release in the US to rapturous reviews.
It is based on the true story of Solomon Northup, who lived a decent, free life in Saratoga Springs near New York until he was duped and kidnapped and transported south to be a plantation slave.
It is often a beautiful and uplifting film but does not flinch from showing the breathtaking cruelty of the slavers. The indignities, beatings and whippings are almost unbearable to watch and will stay with people for a long time.
Chiwetel Ejiofor, like McQueen London-born, gives a blazing performance as Northup. Asked a question about whether he had any concerns about some of the scenes he said: "You can't tell a story about slavery unless you tell it."
The film is now a leading candidate for Oscar success and US critics have been overwhelmingly positive in their praise. The Wall Street Journal called the film "a landmark event … it seems certain to transcend the movie realm and become a new reference point in contemporary culture – a defining vision of what slavery looked like, and felt like, in the US before the civil war." David Denby in the New Yorker called it "easily the greatest feature film ever made about American slavery".
McQueen said it was "a world story because it has to do with slavery and slavery was a world industry".
Helped by an abolitionist – played by Brad Pitt, a producer and financial backer of the film – Northup did eventually get back to his family and wrote an account of his ordeal on which the film is based. McQueen said: "I couldn't believe I hadn't come across this story before. I felt really stupid, but then no one else had."
Ejiofor said 97% of the people making the film were American but there are star turns from the British actor Benedict Cumberbatch as a more benevolent plantation owner and the Irish-German actor Michael Fassbender as an unhinged one.
Fassbender's performance has attracted particular praise but the actor this week said he was not prepared to jump through all the Oscar campaign hoops that Hollywood expects. McQueen said anybody wanting to see Fassbender's Oscar campaign just had to watch the film.
12 Years a Slave has already won the People's Choice award at the Toronto film festival, which is a good omen as both Slumdog Millionaire and The King's Speech – both Oscar winners – won the same award.
McQueen has gone from being one of the UK's leading visual artists – he won the Turner prize in 1999 – to being one of its most talented film directors following his films Hunger (2008) about the IRA prison hunger strikes and Shame (2011) about a desperately unhappy sex addict.
The film is being shown as the London film festival draws to a close. Its awards will be given out at a ceremony on Saturday night and it will close as it opened – with a Tom Hanks film. Saving Mr Banks, in which he stars as Walt Disney, will get its premiere on Sunday.