U-66, the lead boat of the class, was laid down on 1 November 1913 (yard number 203) by Germaniawerft at Kiel and launched on 22 April 1915. She was commissioned on 23 July 1915 under the command of Kapitänleutnant Thorwald von Bothmer.[8] During the war, she sank 25 ships totaling 69,967 gross register tons (GRT).