They were designed with the purpose of laying small minefields and transporting naval special forces, with or without their submersibles, in shallow waters that were inaccessible for larger submarines.

Due to their mission profile that called for a small design as well as the need to stay undetected, they lacked torpedo armament and a generator for battery recharging.

During the Croatian War of Independence and the dissolution of SFR Yugoslavia, five of the six boats were relocated to Boka Kotorska where they served with the SR Yugoslav Navy.

The project, designated B-91 and led by Lieutenant Colonel Davorin Kajić, was tasked with developing a class of midget submarines capable of operating in the shallow waters of the northern Adriatic.

Using their small dimensions, the submarines were to be able to maneuver in depths as shallow as 10 metres (33 ft); such abilities were needed to conduct offensive minelaying near the enemy coastline, reconnaissance, and transporting naval special forces in hostile waters.

[1][2] All six boats were completed from 1985 to 1989 at the Brodogradilište Specijalnih Objekata (BSO) in Split, SR Croatia and named after rivers in SFR Yugoslavia: Tisa (P-911), Una (P-912), Zeta (P-913), Soča (P-914), Kupa (P-915) and Vardar (P-916).

[3] Further planned improvements included the addition of a Stirling engine, either by refitting the existing boats or building a new, seventh one, but the imminent breakup of Yugoslavia happened before anything was realized.

The class lacked the ability to recharge during surface drive making them dependent on external power sources such as harbours and depot ships.

[17] Although the "Submariner" society from Pula was interested in accepting the donation and exhibiting it in a museum, the Croatian MoD responded with a statement that accepting the donation is against national interests; since most of the fleet of the former Yugoslav Navy was relocated to Montenegro at the beginning of the Croatian War of Independence, Croatia claims the military equipment of the former joint armed forces was stolen, and should be a subject of succession.