Bertha Muzzy Sinclair or Sinclair-Cowan, née Muzzy (November 15, 1871 – July 23, 1940), best known by her pseudonym B. M. Bower, was an American author who wrote novels, fictional short stories, and screenplays about the American Old West. Her works, featuring cowboys and cows of the Flying U Ranch in Montana, reflected "an interest in ranch life, the use of working cowboys as main characters (even in romantic plots), the occasional appearance of eastern types for the sake of contrast, a sense of western geography as simultaneously harsh and grand, and a good deal of factual attention to such matters as cattle branding and bronc busting." She was married three times: to Clayton Bower in 1890, to Bertrand William Sinclair (also a Western author) in 1905, and to Robert Elsworth Cowan in 1921. However, she chose to publish under the name Bower.

Bower began writing to "save my sanity" after moving to Big Sandy with her first husband. Seeking financial independence from Clayton, she began sending stories to publishers in 1900. She regularly wrote new material while continuing to send out her old stories once a month. Bower published her first short story, Strike of the Dishpan Brigade, locally in 1901. Her first short story to be published nationally, Ghost in the Red Shirt, appeared in Lippincott’s Magazine in 1904.

Later that year, Bower published her first Western novel, Chip of the Flying U, as a serial in Popular Magazine by Street & Smith. The book introduced readers to the fictional Flying U Ranch and the "Happy Family" of cowboys who lived there. The book was so popular that it was re-released in hardcover in 1906 with three watercolor illustrations by famed painter Charlie Russell. Chip of the Flying U rocketed Bower to fame, and she wrote an entire series of novels set at the Flying U Ranch.

Bower went on to write 57 Western novels. She died in July 1940 in Los Angeles, California, at the age of 68. By the time of her death, her books had sold more than two million copies, not counting her many short stories and articles. According to Elmer Kelton, Bower's sales dropped when it was revealed that she was female.