George MacKay and "Game of Thrones"' Dean-Charles Chapman will star in the war movie, while Cumberbatch's "Sherlock" co-star Andrew Scott, Richard Madden, Adrian Scarborough, and Daniel Mays have also been attached.
Brit Mark Strong is another new addition to the cast added this week, beg March 25, according to Deadline.
Director Mendes co-wrote the screenplay with Krysty Wilson-Cairns. He will also produce with Pippa Harris and Jayne-Ann Tenggren.
The moviemaker has recruited a few old friends for the film, including his revered "Skyfall" cinematographer Roger Deakins, and "Road to Perdition" production designer Dennis Gassner, as well as composer Thomas Newman, who has created the scores for six Mendes films, including "Skyfall" and "American Beauty".
The film follows the fortunes of two young soldiers over the course of one day during the first World War, which raged through Europe from 1914 to 1918.