The production had a fake working title, "Highway 66", so that the shoot of the controversial novel would not be affected by union problems. Much of the dire straits portrayed in the film continued during and after the release of the movie.
While filming the Joads' car traveling down the highway, John Ford wanted to add a shot showing the large number of caravans heading west, so the film's business manager stopped actual cars making the trek and paid the drivers five dollars to escort the Joads' jalopy for the cameras.
Prior to filming, producer Darryl F. Zanuck sent undercover investigators out to the migrant camps to see if John Steinbeck had been exaggerating about the squalor and unfair treatment meted out there. He was horrified to discover that, if anything, Steinbeck had actually downplayed what went on in the camps.
The novel's original ending was far too controversial to be even considered for a film in 1940. It involved Rose-of-Sharon Rivers (Dorris Bowdon) giving birth to a stillborn baby and then offering her milk-filled breasts to a starving man, dying in a barn.
Henry Fonda, still struggling to became a big Hollywood star, tried to avoid being a contract player for 20th Century-Fox because he wanted the ability to independently choose his own projects (an increasing number of stars at the time were trying to gain such independence). But when the much-coveted part of Tom Joad was offered to him, Fonda hesitantly gave in and signed a contract to work with the studio for seven years because he knew it would be the role of a lifetime.
Darryl F. Zanuck paid $100,000 for the rights to John Steinbeck's novel - a staggering amount of money at the time. Steinbeck only allowed the rights to be sold under the proviso that the filmmakers should show the material due reverence and treat the project responsibly.
Although the script conformed to the provisions of the Production Code, a number of potential "problems" had to be addressed. The list of suggested alterations or eliminations included a warning "not to characterize Muley as insane", the rewording of "certain of the lines which have reference to Rosasharn's pregnancy" (in the book, Tom teases Rosasharn and Connie with the line, "Well, I see you been busy"; in the film this is changed to, "Well, I see I'm gonna be an uncle soon"), the removal of a "toilet gag about Grandma" (early in the family's journey Rosasharn leads her out of a gas-station washroom, explaining, "She went to sleep in there"), the elimination of "specific mention of Tulare County [California]" and a request not to identify a town as "Pixley" (a town in Tulare County, CA, notorious for its ill treatment of migrant workers). It was also suggested that the film not show "Tom killing the deputy in self-defense".
Banks and the large farming corporations that controlled most California farms were not keen on the original novel (it was banned in some states and in several counties in California, and the book was not carried in the municipal library of author John Steinbeck's home town of Salinas, California, until the 1990s) and were even less thrilled that a film was being made of it. The Associated Farmers of California called for a boycott of all 20th Century-Fox films, and Steinbeck himself received death threats.
Henry Fonda currently holds the record for the longest gap between acting Oscar nominations. His first nomination was for The Grapes of Wrath (1940) in 1940, his second was for On Golden Pond (1981) in 1981, 41 years later. He received one other Oscar nomination in the period between his two acting nominations, that was for producer of 12 Angry Men (1957) in 1957.
Noah Joad simply vanishes after the scene of the family swimming in the Colorado River. In the book, Noah tells Tom he has decided to stay by the river. In the film, his disappearance is never explained.
Beulah Bondi was tested for the role of Ma Joad. Bondi, believing that she had the part, reportedly bought an old jalopy and moved to Bakersfield (CA) to live among the migrant workers in order to research the role.
Producer Darryl F. Zanuck knew that Henry Fonda was desperate for the part of Tom Joad, so he let it be known that he was going to offer the part to Tyrone Power. Fonda pleaded with Zanuck for the part, and in order to get it Zanuck talked him into signing an eight-picture deal with 20th Century-Fox.
Darryl F. Zanuck was heavily involved in all aspects of the production, as he saw it as a personal project. In fact, so meticulous and carefully thought-through was his editing of Nunnally Johnson's screenplay that Johnson himself praised Zanuck for his attention to detail.
In the book, John Steinbeck had the character of Casy parodying the song "Yes Sir, That's My Baby" by singing "Yes sir, that's my Savior/Jesus is my Savior/Jesus is my Savior now." The Motion Picture Production Code then in effect forbade use of the words "God" and "Jesus" except when used "reverently", so the script resorted to having him hum: "Mm-mmm, mmm, my Savior".
Henry Fonda kept the hat he wore in the movie for the rest of his life, until before he passed away in 1982 he gave it to his old friend Jane Withers. Apparently he and Withers, when she was an 8 year old girl and he a young man, did a play together before Fonda made movies. Fonda was so nervous to go onstage that little Jane took his hand, said a little prayer to ease his nerves, and the two of them became good friends for life.