No film in Hayao Miyazaki's repetoire better reflects his love for aviation than Porco Rosso. Most of his films possess some element of it, some even quite strongly, but none are quite as involved as this paean to the Golden Age of Flight. In fact, it is so involved that the film was underwritten by Japan Airlines!
The story originated in Miyazaki's manga short The Age of the Flying Boat in Japan's Model Graphix magazine, which has since been republished in Hayao Miyazaki's Daydream Note. His manga stories for this magazine, which was itself for scale-model hobbyists, tended to deal in historical subjects and extrapolations on military and aviation technologies from the war period. Japan Airlines saw The Age of the Flying Boat as a perfect subject for an in-flight short.
Once Miyazaki becomes involved in a project, however, it has the potential to spin off into a grander scope. That is what happened to The Age of the Flying Boat. It quickly evolved into a full feature film, but because JAL partially funded it, Porco Rosso did have its debut onboard long before it premiered in theatres.
The film itself has little to speak of, insofar as Miyazaki's usual political, spiritual, philosophical and environmental themes are concerned. Our story revolves around Marco Pagot, a former World War I fighter ace in the Italian Air Force who turned his back on the country after it turned to facism. Rather than serve under Moussilini, he occupies his time as a bounty hunter chasing air pirates. Also, at some undisclosed point in the past, he turned into an anthropomorphic pig.
Vexing viewers since 1992, I believe that the question of why Marco is a pig is overthought. The answer is not found in the film. On the contrary, we must look outside the film to how Miyazaki uses the image of pigs in his other work. They show up, for example, in the Studio Ghibli Museum short Imaginary Flying Machines, which also eventually made it onto JAL flights. There, everyone in the worlds inspired by Jules Verne, Fritz Lang and Albert Robida are pigs. Swine feature most prominently in Miyazaki's own cartoonish depiction of himself. In everything from napkin doodles to the theatrically-released, self-referential Ghiblies 2 short, Miyazaki always draws himself as a pig.
While I am sure that an in-film rationale for Marco's appearance can be found, I don't think it is really important. Marco is a pig because Miyazaki is a pig and Miyazaki sees himself in Marco. Or, at least, Miyazaki sees himself in this film more definitely and intimately than he sees himself in others. Recently he has begun talking about a sequel to Porco Rosso... The only sequel he has deigned to consider. Central to the prospect, tentatively titled The Last Sortie and set during the Spanish Civil War of 1936-39, is that Marco is an old man passing the torch. Miyazaki describes the project as "a hobby of the old man", which he will only consider braving if Studio Ghibli is on firm footing with its next generation of directors.
There are numerous repeating names and ideas winding out of Porco Rosso. The Studio Ghibli Museum's giftshop, Mama Auito, is named after the film's air pirates. It's fictionalized animation studio exhibit, Piccolo Studios, is named for the aircraft company. Some speculate that the Italian-style term "Piccolo" is a conjunction of aircraft makers Caproni and Piaggio. Porco's plane resembles both the Caproni C-22J and Piaggio P-136. Most interesting yet, the Caproni Ca.309 fighter, which was in active service during World War II, went by another nickname: the Ghibli.
While other Miyazaki films are replete with spiritual and political insights, none seem quite so personal as Porco Rosso. Though it lacks any strong statements of meanings, it is thoroughly indulgent of Miyazaki's great affections.