D1 was the prototype for the first class of diesel-driven submarine, a development which signalled the end of the petrol engine era with its attendant dangers to crews from petrol fumes, and D4 was the first British submarine to be fitted with a gun - a 12-pounder on a mounting housed inside the superstructure.
A significant innovation was the use of saddle tanks to hold the main ballast water. The use of external tanks provided additional inboard space and should have resulted in improved habitability standards. However, with the introduction of a stern torpedo tube (the first in a British submarine), an increased engine size to give greater power and an increased complement in a vessel of the same hull diameter as the C Class, and with only 20ft increase in length, it is doubtful whether the additional space was properly utilized to improve accommodation standards.
