The Hrabri-class were the first submarines to serve in the Royal Yugoslav Navy (KM),[a] and after extensive sea trials and testing they sailed from the UK to the Adriatic coast of Yugoslavia, arriving in April 1928.

Nebojša was overhauled and initially served with RN submarine forces in the Mediterranean Sea as an anti-submarine warfare training boat.

In 1925, King Alexander visited the navy for the first time and several senior naval officers suggested to him that a submarine arm was needed.

In the middle of 1926, the British L-class submarine HMS L53 visited the Adriatic to familiarise Yugoslav naval officers with the class and undergo trials to demonstrate its capabilities.

The two Hrabri-class submarines, named Hrabri (Brave) and Nebojša (Fearless),[4] were some of the first new acquisitions aimed at developing a naval force capable of meeting this challenge.

[6] The British Royal Navy cancelled the order for the two submarines in March 1919, and the partially constructed hulls were launched on 16 June and 2 July respectively to free up the slipways on which they were being built.

In November the hulls were sold by the Royal Navy (RN) to the shipyard, and once the contract with the Yugoslavs was signed they were brought back onto the slipways and completed to a modified design.

[3] The Hrabri-class boats were of a single hull design with a straight stem, circular cross section and narrow pointed stern.

[13] For surface running, the Hrabri-class boats were powered by two Vickers V12 diesel engines designed in 1912 that were rated at a combined 2,400 brake horsepower (1,800 kW) at 380 rpm.

The naval historian Zvonimir Freivogel states that on a 15-day Mediterranean cruise, Nebojša needed 420 kg (930 lb) of replacement screws.

Until the destroyer leader Dubrovnik was commissioned in 1932, the Hrabri-class boats had the heaviest armament of any Royal Yugoslav Navy vessel.

[16] Hrabri and Nebojša were launched on 15 April and 16 June 1927 respectively with tactical numbers 1 and 2, which were painted centrally on each side of the conning tower.

The trial and training phase was extensive, and once it was completed, the boats sailed to Portland where they took onboard their complement of torpedoes, before returning to Newcastle.

They then had a five-day visit to Algiers in French Algeria and a brief stop at Malta before arriving at Tivat in the Bay of Kotor on the southern Adriatic coast on 5 April.

[20] In May and June 1929, Hrabri, Nebojša, Hvar and six 250t class torpedo boats accompanied the light cruiser Dalmacija on a cruise to Malta, the Greek island of Corfu in the Ionian Sea, and Bizerte in the French protectorate of Tunisia.

In early June 1930, Nebojša was exercising her crew at periscope depth "targeting" the Sitnica between Dubrovnik and the Bay of Kotor when she collided with the 2,324-gross register ton (GRT) Yugoslav steamship Pracat.

Nebojša's forward gun was lost and her conning tower was damaged, but neither her saddle tanks or hull were breached, so she was able to safely surface.

[20] In 1932, the British naval attaché reported that Yugoslav ships engaged in few exercises, manoeuvres or gunnery training due to reduced budgets.

[24] In 1933, the attaché reported that the naval policy of Yugoslavia was strictly defensive, aimed at protecting her more than 600 km (370 mi) of coastline.

[25] On 4 October 1934, Hrabri and the Osvetnik-class boat Smeli commenced a training cruise in the Mediterranean involving sailing around Sicily independently and meeting at Bizerte.

[26] In 1937, Hrabri participated in a cruise through the eastern Mediterranean along with Smeli, the flotilla leader Dubrovnik and the seaplane tender Zmaj.

The outbreak of World War II less than a year later meant that the ordered boats were never delivered and the Hrabri class had to continue in service.

[28] When the German-led Axis invasion of Yugoslavia commenced on 6 April 1941, the entire submarine flotilla was docked in the Bay of Kotor.

A storm on 23 April near the island of Antikythera tore off some deck plating and two doors from the conning tower, but she made it to Souda Bay on the northwest coast of Crete on the following day.

One or two days later she resumed her voyage to Egypt accompanied by the British steamship Destro and the Papanikolis, and escorted by the Orjen-class torpedo boats Durmitor and Kajmakčalan.

At 12:20 on 27 April, Nebojša's diesel engines broke down and she completed the voyage to Alexandria on her electric motors alone, arriving at 14:20 the same day.

From 17 June until October, Nebojša operated as a submerged target for anti-submarine warfare training, first for the crews of British destroyers and then the fast minelayers Abdiel and Latona.

[29] These ideas were forestalled by the Cairo mutiny of Yugoslav generals in Egypt, after which almost all of the crew of Nebojša was brought ashore in May 1942 and escorted to a British military camp at Abbassia.

All her ammunition and torpedoes were brought ashore, and only one Yugoslav officer and two or three sailors remained aboard Nebojša as part of a mostly British crew.

[4][5][14] In 2011, to mark the 70th anniversary of the invasion of Yugoslavia, the Military Museum in Belgrade, Serbia, hosted an exhibit which included a flag from the Nebojša.