The board's picks, announced Thursday, are considered the first major harbinger of the Academy Awards. The win boosts the awards hopes for "Slumdog," which is viewed as the independent film most likely to follow "Juno" and "Little Miss Sunshine" in Oscar contention.
Directed by Danny Boyle, the film follows the reflections of a poor boy (Dev Patel) from the slums of Mumbai, India, who ends up on the country's version of "Who Wants To Be a Millionaire." Patel was also chosen for best breakthrough performance by an actor.
"Slumdog Millionaire" also won best film at the British Independent Film Awards. With a cast of largely local talent, it was shot in Mumbai, which last week suffered a bloody terrorist attack.
"We are thrilled that we can hopefully bring some positive light to what has happened over there by honoring this film," said NBR President Annie Schulhof.
In an earlier interview, Boyle was grateful for awards attention considering the difficulty many independent films are experiencing.
"It's fantastic for us to have had this kind of platform," said the director, whose previous films include "Trainspotting" and "28 Days Later." "It gets journalists all perked up. It works in so many different ways. It's an amazing vehicle for this kind of film."
Awards were generally spread around to a number of films.
Clint Eastwood was selected as best actor for his performance in the not-yet-released "Gran Torino." Best actress went to Anne Hathaway for "Rachel Getting Married."
David Fincher won best director for "The Curious Case of Benjamin Button." Best adapted screenplay was shared by Eric Roth for "Benjamin Button" and Simon Beaufoy for "Slumdog."
Josh Brolin took best supporting actor for his role in "Milk," and Penelope Cruz won for her supporting performance in Woody Allen's "Vicky Christina Barcelona."
The cast of "Doubt" - headlined by Meryl Streep and Philip Seymour Hoffman - won for ensemble. Viola Davis, also from "Doubt," won for breakthrough performance by an actress.
"Man on Wire" won for best documentary, "WALL-E" for animated feature and "Mongol" for foreign-language film.
The National Board of Review, which is composed of film historians, students and educators, was founded in 1909. The awards will be handed out Jan. 14 in a New York gala hosted by Whoopi Goldberg.
Last year's pick for best film, "No Country for Old Man," went on to win best picture at the Oscars.
By Jake Coyle