The SS Thistlegorm was built by Joseph Thompson & Sons shipyard in Sunderland for the Albyn Line and launched in April 1940. She was powered by a triple-expansion steam engine rated to 1,850 hp (1,380 kW). The vessel was privately owned but had been partly financed by the British government and was classified as an armed freighter. She was armed with a 4.7-inch (120 mm) anti-aircraft gun and a heavy-calibre machine gun attached after construction to the stern of the ship.