Twin-hull flying boat, thick-wing monoplane, with two engines in tandem.

The SIAI S.55 was an all-wood, high-wing cantilever monoplane with a pronounced dihedral, a flying boat (or catamaran) configuration, and a cockpit sunk into the wing's thickness. 

The arrangement allowed for the perfect suspension of torpedoes or bombs between the two hulls, wide fields of fire for weapons placed at their ends, and large internal volumes for passengers or other cargo.

The Air Force was equipped with about 200 examples of this aircraft.

It was also used for long-distance flights and speed races.

Today, only one example is on display at the TAM Aviation Museum in São Carlos, Brazil.