While the plane depicted in the film never existed, Savoia was an actual Italian aircraft maker which produced a considerable number of flying boats in the 1920s, during which the film is set. An actual Savoia S.21 even existed, though the fictional one does not closely resemble it—the Cant.Z 501 "Seagull" is probably the closest real-life match. As a matter of fact, Hayao Miyazaki is said to have drawn his Savoia S.21 from his childhood memories of Macchi M.33.
The S.21 was a custom-built fighter seaplane with a single parasol wing, above which was mounted a single engine nacelle. It had a flying-boat hull and outrigger floats, and carried two machine guns in the nose. In the film, there are two versions of the S.21. The initial version was powered by an Isotta-Fraschini Asso liquid-cooled V-12 engine; the later version mounted a Fiat A.S.2 "Folgore" V-12 with a modified radiator configuration. In addition to the engine, the new version had a tiny forward cockpit.
In The Age of the Flying Boat, the book on which the film is based, the modified version takes a Rolls-Royce Kestrel engine. The aircraft was painted bright red with Italian tricolor stripes on the wings and tail.
- Crew: initial version, 1 pilot; second version, additional 1 passenger
- Length: 7.92 m
- Wingspan: 10 m
- Powerplant: 430 hp (320 kW)
- Maximum Speed: 330 km/h
- Armament: 2 × 7.92 mm Spandau machine-guns