The intent of Toho was that this was going to be the final Godzilla movie as the popularity or Kaiju films was losing steam. However, it was very successful and Toho changed their mind and went on to create more Godzilla films into the 70's.
Until Godzilla: Final Wars (2004), this movie featured more monsters than any other in the Godzilla series, with eleven total: Godzilla, Mothra, Rodan, King Ghidorah, Anguirus, Minya, Spiega (aka Kumonga), Baragon, Gorosaurus, Manda, and Varan!
The "friendly" look continued in Godzilla's appearance from 1968 to 1972. Godzilla, by this time, was perceived as the "hero of the world in his movies instead of an indestructible demon from the nuclear age. This Godzilla suit went on a diet by being slimmed down a lot to fit the actor better, and for better mobility while filming Godzilla's battles. Godzilla's neck became much longer and his head more rounded. His pupils were made a little smaller and the width of his mouth became much longer, traveling down to the upper part of his neck. His dorsal fins became smaller in size and, like the head, was also rounded. His two end dorsal fins became much smaller, and appear as mere little bumps traveling down the back of the suit. The tail was also made shorter.
During the big 4 monster attack on Tokyo (by Godzilla, Rodan, Manda and Mothra) there was a few minutes of footage shot of Godzilla and Manda fighting. It was later decided that it was out of character for the monsters to start fighting, as they were under Kilaak control. The scene was immediately scrapped. It has been rumored that the scene was shot on the "spur of the moment," that the fx people were having a bit of fun (having Manda attack Godzilla), and Tsubarya just let the cameras roll during this. Planned or not, footage of this still exists and had been shown many times at fan conventions.
This was the last Godzilla movie that was made with all 4 "Godzilla Fathers." They are: TANAKA (producer), HONDA (director), IFUKUBE (music) and TSUBURAYA (special effects, although he only worked as an advisor).
In both the Japanese and U.S versions Gorosaurus is accidentally called Baragon when he is attacking France. Baragon was supposed to do that scene. In fact Gorosaurus was given Baragon's roar and was burrowing underground, which is an ability Baragon has, not Gorosaurus! As well in Japan, a painted children's record book based on the movie features Baragon destroying France. The reason for the mix up is that the Baragon costume was on loan to TSUBURAYA Productions to be redressed as various ULTRAMAN monsters and wasn't ready in time for filming. So, Gorosaurus was substituted for that scene.
Destroy All Monsters brought Ishirô Honda back as director and Akira Ifukube as the musical composer for the first time in three years. The previous two Godzilla movies, Godzilla vs. the Sea Monster, and Son of Godzilla, were directed by Jun Fukuda, while the score was composed by Masaru Sato. During this time period, producer Tomoyuki Tanaka wanted to see if another team was capable of directing kaiju movies. The installment after Destroy All Monsters, Godzilla's Revenge, would be Honda's last time directing the series until its last entry, Terror of Mechagodzilla (1975).
Even though a new Godzilla costume was designed for this film, the one used for the attack on New York is the one that was used for Invasion of Astro-Monster (1965) and Godzilla vs. the Sea Monster (1966). Older Godzilla suits were used in certain "stunt" scenes (particularly in scenes with Godzilla in water) to keep the newer suits in shape.
As an apparent inside gag, the television broadcast showing the humans under Kilaak control warns the viewers to "Be on the lookout for Andrew Hughes, an American." Andrew Hughes is the name of one of the other actors. He is the older gentleman playing Dr. Stevenson.
Originally there were plans to have two other monsters in the movie as well as the eleven that were used. Those monsters were to have been Ebirah from Godzilla vs. the Sea Monster (1966) and the giant walrus Maguma from Gorath (1962)
Toho decided to include monsters that have not been seen in a while, along with some of their other monsters whose movies were not connected to the Godzilla series. Anguirus had made his first appearance since "Godzilla Raids Again" (1955); Baragon was last seen in "Frankenstein Conquers the World" (1965); Gorosaurus starred in "King Kong Escapes" (1967); Manda was in "Atragon" (1963) and Varan starred in "Varan the Unbelievable" (1958).
The film in the planning stage was called "Kaiju Chushingura" or "Monster Chushingura". "Chushingura" is a very popular Kabuki plot based on a real story of a band of Samurais taking revenge of their master who was killed. The name of the antagonist of "Chushingura" is "Kira" which was merged with "Aku" which is the Japanese word for "evil". "Kira-Aku" was respelled as Kilaak in the English translation.