Bradford’s greatest son thought the scripts he turned out for the big film studios in the 1930s, to help pay his wife’s health bills, were beneath him.
In fact, according to one expert, they may have made him the true father of the modern horror film, and influenced countless other genres.
But because he nearly always refused an on-screen credit, the extent of his input may never be known.
“The truth may be hidden away in a box in a dusty studio archive somewhere,” said Bill Lawrence, a film historian, who is behind a 90th anniversary screening of one of Priestley’s better-known films in Yorkshire next weekend.
The 1957 musical version of The Good Companions, the novel that established him as a national figure in 1929, is being shown in Halifax, the home town of its star, the Shakespearean actor, Eric Portman.
The book was filmed twice and later adapted into a series by Yorkshire Television. But it was the first screen incarnation, starring John Gielgud and Jessie Matthews in 1933, that brought Priestley to Hollywood’s attention.
“It’s a measure of his stature that he was asked to write for Gracie Fields, the biggest star of her day,” said Mr Lawrence.
Priestley co-wrote her biggest hit, the morale-boosting depression-era musical, Sing As We Go. But when he crossed the Atlantic, he assumed a lower profile.
Wintering on a ranch in Arizona, where the climate suited his second wife, Jane Wyndham-Lewis, he took on work for the film studios over the border in California,.
“He just regarded it as hack work, frankly,” said Michael Nelson, of the JB Priestley Society in Yorkshire.
“He needed to support his wife and children, so he would go over to Hollywood and pick up script work on a freelance basis – on the understanding that he wouldn’t be listed in the credits because he wanted to preserve his reputation as a superior dramatist.”
Back in the UK, his name untarnished by his brush with American popular culture, he began to be held in even higher esteem when he began broadcasting a series of “fireside chats” on the radio which were widely credited with strengthening morale during the Battle of Britain.
Priestley’s presence in Hollywood was not a secret – his meetings with Groucho Marx, Charles Chaplin and others are documented. But with one exception – a 1936 Carole Lombard vehicle called The Princess Comes Across – the films he wrote are not.
“It’s quite tantalising. Priestley mentioned a few producers but his papers don’t tell us which studios he worked for,” Mr Nelson said.
“Given Hollywood’s output in the 1930s it would be nearly impossible to discover everything he did.”
But Mr Lawrence, a former head of film at Bradford’s National Science and Media Museum, said it was clear that his influence extended far beyond his signature works.
“Priestley is one of the major literary figures of the 20th century but his contribution to cinema is often overlooked,” he said.
“But the more I looked into him, the more I was intrigued by his position right across the board, culturally.
“His involvement goes right back to his 1927 novella, Benighted, which became the film, The Old Dark House. That started a whole new horror genre of strangers seeking shelter in an isolated house full of strange people.
“It’s really the basis for The Rocky Horror Picture Show.”
Priestley’s archive continues to be raided for film and TV. Last Holiday, his 1950 comedy starring Alec Guinness, was remade in 2006 with the American actress, Queen Latifah. And an episode of the BBC’s Doctor Who last year was “based on an original idea” by him.
• The screening of The Good Companions at Halifax’s Chapel Arts Centre next Sunday will mark not only the 90th anniversary of the book on which it was based but also the 50th of the death of its star, the actor Eric Portman.
Born just a mile from the Arts Centre, in the suburb of Akroydon, he was known for such war movies as The Colditz Story and One of Our Aircraft is Missing. His Technicolor remake of The Good Companions had a stellar cast which also included Celia Johnson, but it was less well received than the 1933 original.
Priestley was born in Bradford’s Manningham district, the son of a headmaster. A statue of him stands in the city centre.