Porco's aircraft, according to the movie, was an experimemental aircraft captured by Porco during the Great War and instead of scrapping it, he decided to make it his own aircraft and painted red. The aircraft's name is the Savoia S.21, though it looks nothing like it (Savoia S.21 is an real aircraft).
His aircraft's fuselage is round and has a high wing with an engine on top of it. The wing is slightly swept back (more swept in Fio's later design) and is placed unusually forward. His aircraft is also armed with 2 forward firing machineguns.