The first three U Class submarines were ordered in 1936 to serve as unarmed targets for anti-submarine vessels. But, in a change of policy (perhaps with a foreboding of war) Undine, Unity and Ursula, laid down at Vickers in February 1937, were modified during construction to accommodate six bow tubes (four internal and two external) so that they could undertake short offensive missions. To allow the installation of a small deck gun, the hull forward of the conning tower was reinforced.
