The five boats were sold to the Imperial Germany Navy at the beginning of World War I when it was thought impossible for the submarines to reach the Mediterranean for delivery to Austria-Hungary.

After the outbreak of World War I in August 1914, the Austro-Hungarian Navy became convinced that delivery of the still-unfinished submarines to the Mediterranean via Gibraltar would be impossible.

The German Navy assigned the numbers U-66 to U-70 to the five submarines and had them redesigned and reconstructed to their specifications.

In 1904, the Austro-Hungarian Navy, after allowing the navies of other countries to pioneer submarine developments,[4] ordered the Austrian Naval Technical Committee (German: Marinetechnisches Komitee or MTK) to produce a submarine design.

They were looking for a double-hulled submarine of about 500 tonnes (490 long tons) displacement with diesel propulsion.

They also wanted a surface speed of 16–18 knots (30–33 km/h; 18–21 mph), and for the boat to be armed with between three and five 45 cm (17.7 in) torpedo tubes.

The Austrian specifications called for two shafts with twin diesel engines (2,300 metric horsepower; 2,269 bhp; 1,692 kW total) for surface running at up to 17 knots (31 km/h; 20 mph), and twin electric motors (1,240 PS; 1,223 shp; 912 kW total) for a maximum of 11 knots (20 km/h; 13 mph) when submerged.

[3][Note 3] As a result, the five boats of the class were transferred to the Imperial German Navy on 28 November 1914 after the advance payment of 2 million Kronen had been returned.

[6] The boats were all completed and commissioned into the Imperial German Navy between July and September.

[6] U-68 was sunk by the British Q-ship HMS Farnborough in March 1916, and was the first boat of the class to be lost during the war.

[1] 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.

[16] During her service career, U-69 sank 31 ships with a combined gross register tonnage of 102,875,[17] the largest among them, the 13,441 GRT armed merchant cruiser Avenger.

Her fate is officially unknown, even though British reports credit her sinking to destroyer HMS Patriot.

[20] During the war, Wünsche and U-70 sank one warship, the British sloop HMS Rhododendron, and 53 civilian ships totaling 137,774 GRT.