In the Owaranai Hito Miyazaki Hayao (Never-Ending Man: Hayao Miyazaki) special that aired in November 2016, Miyazaki reported that he wanted to return to making an anime feature film after his “Kemushi no Boro” short for the Ghibli Museum (which debuted on March 21).
Studio Ghibli producer Toshio Suzuki reported in April 2017 that Miyazaki has been drawing the storyboards for the project since July 2016.
Despite not officially receiving a green-light for the feature film at the time, Miyazaki decided to start animation work on the project anyway.
Miyazaki himself had stated last October that he would need three or four years to complete his action-adventure fantasy film.