Piaggio P.108
The Piaggio P.108 was Italy's only four-engine heavy bomber of the Second World War, an all-metal design by Giovanni Casiraghi that first flew on 24 November 1939 and entered Regia Aeronautica service in 1941. Production was very small and is accounted differently by the sources used here: both Wikipedia articles count 24 P.108B bombers and a single P.108A anti-ship aircraft, while Italian Wikipedia gives a combined total of 35 aircraft across all versions and prototypes, against the higher figures in English Wikipedia's infobox and text. The bombers served with a single squadron on night raids against Gibraltar, Algeria and Sicily, and the P.108T transports passed to the Luftwaffe, which flew nine of them until 1945.