The G.12 was an all-metal low-wing cantilever personnel transport aircraft. It had three radial engines, one mounted on the nose and the other two in wing-mounted nacelles. The engines drove three-blade feathering metal propellers. The mainwheels of its landing gear retracted into the nacelles; the tailwheel was fixed. The flight deck and cabin were fully enclosed. Access was via a portside access door aft of the wing.
The G.12 was designed as a civil aircraft, but served mainly in military roles during the war. Only a limited number were built, some as late as 1944, after the Italian armistice. The G.12 inspired the postwar G.212 "Flying Classroom", the last Italian three-engine transporter. It had a crew of four.