The Otra and the Rauma were two purpose-built minesweepers constructed at Nylands Verksted in Oslo. Both ships were completed and commissioned only a short time before the German invasion of Norway. The Otra class vessels used the Oropesa system of mine sweeping. As the threat of war in Europe became ever more clear the decision was made to improve the Royal Norwegian Navy's mine warfare capabilities. At first a number of 2. class gunboats were rebuilt into minelayers and mine sweepers, but with war looming it soon became clear that more capable vessels were required.