Takeo Murata and Ishirō Honda; story by Shigeru Kayama
Movie screenwriting is a "lot of cooks in the kitchen" type scenario. For that matter, there's a fair number of kitchens in the mix, too. The director, the producers, the actors, and even the effects guys can all have a say in the final product, depending on what they are looking for, what they think is best for the film, or simply what they can deliver with the resources available.
Oh, did we forget to mention screenwriters?
Sometimes even they get to have a say in the movie's story.
Bottom line: it can be difficult to determine who provided what part of the Godzilla the world saw on the screen. But Shmoop is here to serve, so here we go.
Godzilla began life in the head of Tomoyuki Tanaka. A producer at Toho Studios, Tanaka noticed the growing popularity of monster movies and decided to get in on that action. He hired Shigeru Kayama, a mystery writer, to pen the original story. According to Steve Ryfle, there was a lot still in development at this point, and Kayama received input from Tanaka and other officials at Toho as he went (source).
For example, Godzilla wasn't a prehistoric creature in Kayama's original treatment. While the shape of the creature didn't solidify until later, the working image appears to have been a sea monster that was a cross between a whale and a gorilla. This is where Godzilla's Japanese name, Gojira, came from (it's a cross between the Japanese words for "whale" and "gorilla"). While the creature ultimately changed into the reptilian monster we know and love today, the name stuck.
Kayama's characters were also more eccentric than the ones we see. Yamane, for example, was a loner character who wore dare shakes and a cape and only came out at night (Source).
Kayama's treatment was then given to Takeo Murata and Ishirô Honda to be fleshed out and written into a complete script. The duo mostly focused on developing the characters and subtext. They toned down the main characters, making them more everyday folk. They also introduced the love triangle, reduced Shinkichi's role, and added scenes like the train commuters.
They began the process of changing the titular beast, giving Godzilla a prehistoric dinosaur form more in line with Tanaka's original inspiration, The Beast from 20,000 Fathoms. Honda also pushed back the big reveal of the beast. In the original draft, Godzilla appeared during the typhoon, but Honda chose to build suspense by saving letting the mystery linger a bit longer (Source).
And that's how Godzilla went from the head of a producer to a draft written by a well-regarded writer to the hands of several more writers. After that, Honda and special effects supervisor Eiji Tsuburaya likely tweaked the story further based on the reality of shooting a film, and the end result is, well, Godzilla.