Answer by taking a quote from the following article:

Howard Robard Hughes Jr. (December 24, 1905 - April 5, 1976) was an American business magnate, investor, record-setting pilot, film director, and philanthropist, known during his lifetime as one of the most financially successful individuals in the world. He first made a name for himself as a film producer, and then became an influential figure in the aviation industry. Later in life, he became known for his eccentric behavior and reclusive lifestyle--oddities that were caused in part by a worsening obsessive-compulsive disorder (OCD), chronic pain from several plane crashes, and increasing deafness. As a maverick film tycoon, Hughes gained prominence in Hollywood beginning in the late 1920s, when he produced big-budget and often controversial films like The Racket (1928), Hell's Angels (1930), and Scarface (1932).

During the 1940s to the late 1950s, the Hughes Tool Company ventured into the film industry when it obtained partial ownership of the RKO companies which included RKO Pictures, RKO Studios, a chain of movie theaters known as RKO Theatres and a network of radio stations known as the RKO Radio Network.  In 1948, Hughes gained control of RKO, a struggling major Hollywood studio, by acquiring 25% of the outstanding stock from Floyd Odlum's Atlas Corporation. Within weeks of acquiring the studio, Hughes dismissed three-quarters of the work force and production was shut down for six months during which time investigations were conducted of each employee who remained with RKO as far as their political leanings were concerned. Only after ensuring that the stars under contract to RKO had no suspect affiliations would Hughes approve completed pictures to be sent back for re-shooting. This was especially true of the women who were under contract to RKO at that time. If Hughes felt that his stars did not properly represent the political views of his liking or if a film's anti-communist politics were not sufficiently clear, he pulled the plug. In 1952, an abortive sale to a Chicago-based group connected to the mafia with no experience in the industry also disrupted studio operations at RKO even further.  In 1953, Hughes was involved with a high profile lawsuit as part of the settlement of the United States v. Paramount Pictures, Inc. Antitrust Case. As a result of the hearings, the shaky status of RKO became increasingly apparent. A steady stream of lawsuits from RKO's minority shareholders had grown to be extremely annoying to Hughes. They had accused him with financial misconduct and corporate mismanagement. Since Hughes wanted to focus primarily on his aircraft manufacturing and TWA holdings during the Korean War years, Hughes offered to buy out all other stockholders in order to dispense with their distractions.  By the end of 1954 Hughes had gained near-total control of RKO at a cost of nearly $24 million, becoming the closest thing to a sole owner of a Hollywood studio seen in three decades. Six months later, Hughes sold the studio to the General Tire and Rubber Company for $25 million. Hughes retained the rights to pictures that he had personally produced, including those made at RKO. He also retained Jane Russell's contract. For Howard Hughes, this was the virtual end of his 25-year involvement in the motion picture industry. However, his reputation as a financial wizard emerged unscathed. During that time period, RKO became known as the home of film noir classic productions thanks in part to the limited budgets required to make such films during Hughes' tenure. Hughes reportedly walked away from RKO having made $6.5 million in personal profit.  General Tire was interested mainly in exploiting the value of the RKO library for television programming even though it made some attempts to continue producing films. After a year and a half of mixed success, General Tire shut down film production entirely at RKO at the end of January 1957. The studio lots in Hollywood and Culver City were sold to Desilu Productions later that year for $6.15 million.

What other film stars did he have under contract with RKO?
RKO became known as the home of film noir classic productions thanks in part to the limited budgets required to make such films during Hughes' tenure.