Mitchum was initially known for his work in film noir. His first foray into the genre was a supporting role in the 1944 B-movie When Strangers Marry, about newlyweds and a New York City serial killer. Undercurrent, another of Mitchum's early noir films, featured him playing against type as a troubled, sensitive man entangled in the affairs of his brother (Robert Taylor) and his brother's suspicious wife (Katharine Hepburn). John Brahm's The Locket (1946) featured Mitchum as bitter ex-boyfriend to Laraine Day's femme fatale. Raoul Walsh's Pursued (1947) combined Western and noir styles, with Mitchum's character attempting to recall his past and find those responsible for killing his family. Crossfire (also 1947) featured Mitchum as a member of a group of World War II soldiers, one of whom kills a Jewish man. It featured themes of anti-Semitism and the failings of military training. The film, directed by Edward Dmytryk, earned five Academy Award nominations. Following Crossfire, Mitchum starred in Out of the Past (also called Build My Gallows High), directed by Jacques Tourneur and featuring the cinematography of Nicholas Musuraca. Mitchum played Jeff Markham, a small-town gas-station owner and former investigator, whose unfinished business with gambler Whit Sterling (Kirk Douglas) and femme fatale Kathie Moffett (Jane Greer) comes back to haunt him. On September 1, 1948, after a string of successful films for RKO, Mitchum and actress Lila Leeds were arrested for possession of marijuana. The arrest was the result of a sting operation designed to capture other Hollywood partiers, as well, but Mitchum and Leeds did not receive the tipoff. After serving a week at the county jail (he described the experience to a reporter as being "like Palm Springs, but without the riff-raff"), Mitchum spent 43 days (February 16 to March 30) at a Castaic, California, prison farm. Life photographers were permitted to take photos of him mopping up in his prison uniform. The arrest inspired the exploitation film She Shoulda Said No! (1949), which starred Leeds. The conviction was later overturned by the Los Angeles court and district attorney's office on January 31, 1951, after being exposed as a setup. Whether despite, or because of, Mitchum's troubles with the law and his studio, his films released immediately after his arrest were box-office hits. Rachel and the Stranger (1948) featured Mitchum in a supporting role as a mountain man competing for the hand of Loretta Young, the indentured servant and wife of William Holden. In the film adaptation of John Steinbeck's novella The Red Pony (1949), he appeared as a trusted cowhand to a ranching family. He returned to film noir in The Big Steal (also 1949), where he joined Jane Greer in an early Don Siegel film.

Answer this question "what is the film noir?" by extracting the answer from the text above.
Mitchum was initially known for his work in film noir.