An early draft of the script, called "Planet of the Men", and written by Pierre Boulle in his native French, featured a messianic Taylor 14 years after the events of Planet of the Apes (1968). It involved an uprising against the apes, following which they revert back to their primal states.
Despite the original being a significant success, the budget was slashed for this sequel. It went from $5 million to $2.5 million in one fell swoop. This was mainly due to 20th Century Fox teetering on the brink of bankruptcy following some majorly expensive failures, such as Hello, Dolly! (1969), Star! (1968) and Tora! Tora! Tora! (1970) The recruiting of Ted Post as director was instrumental here as he was used to minimal budgets and shortened schedules from his work on television.
The only film in the original series of five that does not star Roddy McDowall, who was committed to another project. Archive footage of McDowall as Cornelius is played at the start, and David Watson plays the character in the film proper. Despite this, McDowall is often pictured on video and DVD packaging for this film.
Originally, there was going to be a scene featuring a half-human/half-ape child. However, the producers were afraid that not only would the scene be too confusing, but that they would also lose their "G" rating.
James Franciscus had largely spent most of his career playing doctors and lawyers so welcomed the opportunity to wear a costume that could best be described as minimal. Franciscus was a natural athlete and keen tennis player in real life so was in great shape for the shoot.
20th-Fox Producer Irwin Allen used the tiled tubular set of subterranean New York for an episode of Land of the Giants (1968), and as an electrical power-duct in his TV-movie City Beneath the Sea (1969). In both cases, panels of red lights were inset into the tiled tube making it more useful in these later projects. The same set would be used in Conquest of the Planet of the Apes (1972) as corridors between sections of the Ape Management complex (utilizing white lights).
The 'official' budget is $3 million. However, director Ted Post stated the budget was $2.5 million. Another historian claims it was $4.7 million. All these discrepancies can be found in the Bluray 40 Yr Box Set; under special feature documentaries for POTA; Beneath; and Battle.
Several sources (including the official novelization, comic adaptation, and music CD soundtrack), reveal Brent's first name as 'John'. His dying fellow astronaut, called only Skipper, wears a nametag which, when magnified in freeze frame, appears to be Maddon or Maddox. Don Pedro Colley's character is named "Ono Goro" in the script--Colley was bemused to discover that he was credited as "Negro" in the end titles.
All five original "Planet of the Apes" movies were #1 at the U.S. box office when released. "Beneath the Planet of the Apes" spent two weeks as the #1 top grossing film: the week of May 31, 1970 it made $3,595,936; and the week of June 7, 1970 it made $3,146,395.
The trivia items below may give away important plot points.
Charlton Heston was reluctant to reprise the role of George Taylor for this movie. He eventually agreed on condition that his scenes had to be shot within a two week period. He also insisted that Taylor had to be killed. He agreed to a compromise in which he'd disappear in the beginning of the film and reappear to die at the end. Heston claims in the documentary Behind the Planet of the Apes (1998) that he personally suggested the ending, saying, "Why don't I just set off this bomb and destroy the world. That's the end of the sequels."
An alternate ending was written where Taylor, Brent and Nova escape the underground city prior to the detonation of the bomb, which was not a doomsday device as it is in the finished film. They return to Ape City and, along with Zira and Cornelius, release the humans from the cages and a new order is begun. The script ends hundreds of years later with the Lawgiver teaching a group of ape and human children, who sit in harmony together. The final shot of the script shows a mutated gorilla emerging from the underground and fatally shooting a flying dove. (The "Lawgiver Scene" was resurrected as the framing structure for Battle for the Planet of the Apes (1973).)