Film noir is a cinematic term used to describe Hollywood crime dramas, particularly those that emphasize cynical attitudes and dark characters. Hollywood's classic film noir period is from the early 1940s to the late 1950s but there have been a few modern films that could be described as neo noir, like Brick (2005).
Although Hollywood made a lot of movies about crime and gangsters in the 40s the term “Film Noir” was not a genre or a name people were familiar with until the 50s when Europe was exposed to the American film industry, before then the films we know as noir classics like double indemnity or detour were put into a category called “pulp fiction” which developed the idea of hard boiled anti-heroes, or were thought of as crime thrillers, it was a French film critic called Nino Frank who first coined the phrase Film Noir. It was these genres that would be the building blocks of the noir genre.
During the Second World War a lot of European Jews fleeing from the Nazis settled in America, here is when German expressionism was first adopted and became a major part of the Noir stylistics. 
The representation of women was characteristic of the times of the early 20th century when traditional masculinity was threatened by previously oppressed females taking men's roles in life as bread winners, go-getters and ambition chasers after they got a taste of working life when the men were away at war. In the 1930s, women with such characteristics were seen as dangerous and script writers and directors took advantage of the stereotype and gave the world some leading ladies you wouldn't want to meet in a dark ally.
B-movies were another influence in creating film noir, B-movies were cheaply made and had unknown cast members so they were not as regulated as the more popular A-movies, consequentially directors had more freedom to make darker plots and use their own stylistics.
