The major film distributors in the United States are also the major studios. Even if they've had nothing to do with your production, if you can get your film into the hands of the right people, and if those people believe it will sell well at the box office, they will distribute your film. However, it can be difficult, if not impossible, to simply make an appointment with a studio to screen your film. If you're an unknown with an unknown film in your hands, very few studio executives will be willing to spend any time viewing your project.
One method of getting your film into the hands of studio decision-makers is to transfer your film to DVD, which can be done fairly inexpensively if your editing and cinematography are sound. By duplicating the DVD onto dozens of copies, you can hand DVDs out to virtually anyone you can find who has some sort of access to studio executives. Junior directors, production assistants, secretarial staff, or anyone with a connection can get the ball rolling and get your film noticed.