The first U-28 was a Type U-27 U-boat, ordered to be built on February 19, 1912, launched on August 30, 1913, from Kaiserliche Werft of Danzig, and commissioned into the Kaiserliche Marine on June 26, 1914 with Georg-Günther Freiherr von Forstner in command. Frhr.v. Forstner was relieved on June 15, 1916, by Otto Rohrbeck who was in turn relieved on August 5 by Frhr.v. Loe-Degenhart. On January 15, 1917, Kapitänleutnant Georg Schmidt took command.
U-28 conducted four patrols, sinking 39 ships totalling 93,782 tons. Der Krieg Zur See: Der Handelskrieg mit U-Booten, the official history of the U-boats of World War I, describes U-28's final patrol thusly:
A British description of the same event, Under the Black Ensign by R.S. Gwatkin-Williams, states that when the ammunition detonated, a truck carried as deck cargo was blown into the air and fell from a great height on the U-boat, sinking it. While it is not impossible that a doomed but heroic lorry wreaked vengeance for King and Country, the blast from the explosion probably laid the submarine over far enough to swamp her open hatches.
The second U-28 was a Type VIIA submarine of the Kriegsmarine. Her keel was laid down December 2, 1935, by AG Weser of Bremen. She was launched on July 14, 1936, and commissioned into the Kriegsmarine on September 12, 1936, with Kapitänleutnant Wilhelm Ambrosius in command. Ambrosius was succeeded by nine other commanding officers over the next eight years.
U-28 conducted six patrols, sinking 13 ships totalling 56,272 tons and damaging two others totalling 10,067 tons.
U-28 sank by accident on March 17, 1944, at the U-boat pier in Neustadt, The boat was raised in March 1944 but was stricken on August 4, 1944. The submarine's crew suffered no casualties during her career.