Imperial German authorities were already understanding the value of attack submarines to commerce and warships even before the first shots of World War 1 (1914-1918) were fired in the summer of 1914. As a result, some forty eight boats of thirteen classes were already either on hand on in the works when the fighting broke out between rival powers. U-boats went on to become a vital component of Imperial German Navy actions for the rest of the war, even into its waning days and the ultimate German defeat.
By 1917, thought turned to more technology-laden forms of greater capability and a new design - the "Type 139" - was being developed to cover a long-range, ocean-going requirement. As the United States was set to enter the war in April of 1917), it was evermore imperative to promote German naval prowess as far as the American East Coast and possibly beyond.
Type 139 boats were born under the "Project 46" designation and were to become the largest boats built for German naval service while also being developed from the outset as military cruising submarines. The class would be made up of three total U-boats built to the standard and provide the service with the long-range arm it needed. The program was assigned to Germaniawerft of Kiel in August of 1916 and eventually begat "U-139", "U-140" and "U-141".
Ordered on August 1st, 1916 and built at Yard No. 300, U-139 was launched on December 3rd, 1917 but not officially commissioned until May 18th, 1918.
The vessels were built around a thicker "double-hulled" approach giving the structure greater strength and resilience in the deep. The installed deck guns were aided by a range-finding device which enhanced accuracy and other features included a raised superstructure to better separate it from the pressurized hull in the event of a direct hit to the former. On the whole, the general form and function of the submarine was on par with contemporaries including a flattened deck, boat-like bow, and rudder-controlled stern sporting twin propeller shafts. The sail was set at midships.
Internally, power was derived from a diesel-electric arrangement that involved 2 x Germania marine diesel units, 1 x MAN-Brown Boveri diesel unit, and 2 x AEG electric motors. All told, the powerplant produced up to 3,300 horsepower when surfaced, giving speeds of nearly 16 knots. Submerged travel was limited to just over 7.5 knots. Where the design shined was in its inherent operating range, maximized to 17,750 nautical miles during surface travel at 8 knots - though this fell drastically to just 53 nautical miles when traveling submerged.
The armament scheme involved 4 x 533mm (21") torpedo tubes facing the bow and 2 x 533mm (21") tubes facing the stern - a common practice of 20th Century attack submarines until the end of World War 2 (1939-1945). Up to 24 x G6 torpedoes could be carried aboard. For surface work - the preferred attack method - the boat carried 1x 150mm SK L/45 deck gun facing the bow and 1 x 150mm SK L/45 facing the stern. Armament was rounded out by 2 x 88mm SK L/30 automatic guns which, all told, gave the boats great firepower and capability against all manner of surface threats and commerce-minded vessels.
All three boats were completed before the end of the war and pushed into service at varying degrees. For their short time in service to the Imperial flag, the boats were used throughout the Atlantic. U-139 was able to claim three merchantmen during action in early October 1918 while, on October 14th, 1918, the U-boat engaged the Portuguese 487-ton trawler Augusto de Castillo, leading to a running gun battle which U-139 eventually claimed the victory in. After Castillo's crew abandoned ship, the trawler was taken over and sunk - becoming the last target to be claimed by the much-feared U-boat fleet.
In all, U-139 went on to sink four ships - Bylands, Manin, Rio Cavadao, and de Castillo - and damaged Britain's HMS Perth. With the end of the war in November of 1918, U-139 was passed on to France as a war prize and served the Marine Nationale under the name of "Halbronn" until 1935. Both Japan and the Soviet Union eventually held access to the design plans of the Type 139 boats with the former using it to develop their "Kaidai" boats and the latter passing on the design.
U-140 did the series best as it was able to claim seven total ships, these belonging to owners Portugal, Japan, the United States, and Great Britain. However, it was surrendered on February 23rd, 1919 and unceremoniously sunk as a target on July 22nd, 1921 by its new owners, the United States.
U-141 was launched on January 9th, 1918, commissioned on June 24th, 1918, and surrendered as soon as November 26th, 1918 - just weeks after the cessation of hostilities.
Larger U-boats based in the Type 139 class were planned by the German Navy under the "Project 47" name but this initiative fell to naught due to the end of the war.