Despite Italy's limited participation in the North Atlantic passenger trade before 1914, all three of the country's transatlantic shipping lines made great progress after the Great War ended and, by the mid-1920's, together boasted a sizable fleet of passenger vessels catering for the seemingly endless demand. In 1926 and 1927, Navagazione Generale Italiana (NGI) introduced their 32,500 ton sisters Roma and Augustus and, so successful were they, that the company took the decision to order a new even bigger flagship almost immediately.

The Italian government, conscious of the international promotional value of a sumptuous express liner, agreed to a substantial construction loan as well as a guaranteed operational subsidy, and the order was placed with the Ansaldo Yard in Genoa. NGI and the government agreed at the outset that the new vessel must be not only the acme of Italian art and design but also powerful enough to wrest the coveted "Blue Riband" from whomever held it when she was ready for sea. Once the tonnage had been settled at no less than 50,000, first planning and then building proceeded apace. Originally to be called the Guglielmo Marconi (after the inventor of wireless), Mussolini decided to change the name to Rex in an attempt to curry favour with Italy's royalists and then consolidated his plan by inviting King Victor Emmanuel III to perform the launching ceremony. The finished hull of the largest vessel ever built in an Italian yard slid down the ways on 1st August 1931 and, when completed in September 1932, was registered at 51,062 tons gross. Measuring 880 feet in length with a 97 foot beam, she had handsome accomodation for 2,258 passengers in four classes and carried a crew of 756. Driven by quadruple screws powered by Parsons' geared turbines producing 142,000shp., she was designed to cruise at 28 knots but could make 29 knots if neccessary.

By the time she entered service, her owners NGI had been absorbed into the new Italian Line and it was flying their colours that Rex cleared Genoa on her maiden voyage to New York on 27th Setember 1932. Dressed overall and with a full passenger list of celebrities, including the Mayor of New York, she sailed amidst great acclaim only to break down off Gibraltar where she was forced to dock for embarrassing repairs. Her first eastbound crossing was also dogged by mechanical problems, but the following August she did capture the "Blue Riband" with her westbound passage and set a new record which stood until May 1935 when it was lost to Normandie. Rex remained in service for the first few months of the Second World War but was laid up at Bari, on the Adriatic coast, in the Spring of 1940. That August she was towed to Trieste for the duration of hostilities and on 8th September 1944 was attacked by British aircraft using rockets. Receiving 123 direct hits, she burst into flames and the next day rolled over and sank at her moorings in the shallow waters of Capodistria. When the War ended, plans to salvage her were abandoned because of her severe damage and she was broken up in situ.