The HMC Prince David, Prince Henry and Prince Robert ships were built at Birkenhead in the United Kingdom by Cammell Laird for the Canadian National Steamships Company. By the 1930s they sat idly much of the time because of the decade's decline in trade. When war broke out in September of 1939, the Naval Service lost no time in making arrangements for the conversion of the Princes, adding anti-submarine weaponry for armed merchant cruiser operations.