As Tanaka tells it (as related by Tsutsui), he was working on an expensive Toho production called “Shadow of Honor.” According to the Asia-Pacific Journal, “Shadow of Honor” was an ambitious co-production between Japan and Indonesia, and was to be about a Japanese soldier who stayed in Indonesia after the end of World War II to participate in the Indonesian uprising. It seems, however, that the production fell through when real-life diplomacy broke down between Japan and Indonesia in the mid-1950s. This left producer Tanaka with a big empty hole in the Toho production schedule and a huge sum of money allocated for a film that didn’t seem to be happening anymore.
Also worth noting is that Merian C. Cooper’s and Ernest B. Schoedsack’s 1933 hit “King Kong” had just been re-released for its 20th anniversary, and it proved to be a big hit in Japan. Tanaka seemed to intuit that effects-based outsized animal films were an invitation to large box office receipts. Giant monsters were also cropping up in the United States, as with the 1953 film “The Beast from 20,000 Fathoms.”
So when Tanaka was flying back to Japan from an ill-fated meeting in Indonesia to discuss the falling apart of “Shadow of Honor,” he claims to have had an “Eureka!” moment. When looking down from the plane on the islands below, and pondering the Bikini Atoll and the giant monsters in America, he began thinking (and later said):
“The thesis was very simple. […] What if a dinosaur sleeping in the Southern Hemisphere had been awakened and transformed into a giant by the Bomb? What if it attacked Tokyo?”
Godzilla was, it seems, born at that moment.