[ Call of the Wild ]
Jack London's hugely popular adventure novel, set during the Yukon gold rush, was first published in 1903, and has been filmed multiple times.
The main dog in the story, named Buck, is, in the novel, a St Bernard-Scotch Shepherd mix; the different film versions have used various breeds. Since much of the focus of London's story is Buck's becoming a fierce, semi-feral dog who eventually joins a wolf pack, it doesn't make much sense for a Newfoundland to be used in this role. However, one of the sled-dogs in the novel, Curly, is indeed identified as a Newfoundland, though he is mentioned only a few times and dies halfway through the story.
The 1935 film version, which stars Clark Gable and Loretta Young, takes very great liberties with London's story. One "movie dog" site claims there is a Newf in this movie, but while one of the frequently seen sled dogs is a larger black dog with a long coat, its prick ears and long, tapered muzzle make it look more like a Belgian Sheepdog than a Newf, though if I had to bet, I'd say it's neither. The main dog, Buck, is played by a long-haired Saint. [ IMDB.com page ]
The 1972 version, featuring Charlton Heston and a "spaghetti-Western" feel (although filmed in Norway and Spain) has no "Curly" role and no Newfoundlands; the main dog, Buck, is played by a GSD. [ IMDB.com page ]
The 1997 made-for-TV version, starring Rutger Hauer, has Buck played by a Leonberger; there is no Newfoundland in this movie. [ YouTube page ]