Designed by Ing Giovanni Pegna, the P.3 was a four-engine, two-bay biplane with wings of unequal span, the lower wing being greater in span than the upper.

The P.3's tail was of biplane configuration with three rudders mounted between the two planes.

The crew of four consisted of a pilot, co-pilot, nose gunner, and rear gunner; the pilot and co-pilot sat side by side in a cockpit just ahead of the wings, the nose gunner manned a machine gun mounted in a cockpit in front of them, and the rear gunner manned two machine guns, one in an amidships cockpit behind the pilots and another mounted in a ventral tunnel for defense against attacks from below.

The P.3 made its first flight in 1923, and Piaggio later installed two 306-kilowatt (410-horsepower) Fiat A.20V engines on the aircraft.

However, the Regia Aeronautica (Italian Royal Air Force) did not place a production order for the P.3.