According to one account, to prevent them from falling into German hands, the surviving aircraft and incomplete airframes were destroyed by their crews and factory staff.

It was intended that after this advanced training, they would return to Yugoslavia and be offered specialist roles in the VVKJ or in the aeronautical industry.

Contemporary thinking within the VVKJ led them to evolve their initial ideas into a strut-braced gull-wing monoplane armed with a hub-firing autocannon and fuselage-mounted synchronised machine guns.

A scale model was tested in the Eiffel-built wind tunnel in Paris, but the pair soon realised that they needed a third engineer to help evaluate the design and determine the structural details.

The project name for the IK-2 was changed from IK, standing for (Ljubomir) Ilić and Kosta (Sivčev), to IKZ, to include Zrnić.

The designers favoured manoeuvrability over speed, trying to find a compromise between the German and British concepts of a modern monoplane fighter.

The design had a smaller wing area than the Hawker Hurricane and the Supermarine Spitfire, to achieve a higher speed for the engine power.

[2] The designs were delivered to the VVKJ in time for approval by mid-1936, but a general reluctance to adopt new concepts delayed the IK-3, and a contract for the production of the prototype was not signed until March 1937.

[2] These pilots observed that the controls were highly sensitive; the only real criticisms related to the visual distortion caused by the curved panels of the canopy.

When he reached 400 m (1,300 ft), the windscreen detached from the aircraft; Pokorni pulled up hard and the strain broke off half of the starboard wing.

Modifications were made for the production model, including the use of flat plexiglass panels in the windscreen and the canopy to provide better visibility.

[6] Development of a dual-control two-seat trainer variant of the IK-3 had commenced but pressure on the design team had delayed the completion of the project when the invasion intervened.

[11] In its first year of service, an IK-3 was lost when one of the squadron commanders, Kapetan Anton Ercigoj, was making a mock attack on a Potez 25 over the confluence of the Sava and Danube rivers.

[14] A second wave of German aircraft arrived over Belgrade at 10:00 and the remaining IK-3s were scrambled with the rest of the 51st Fighter Group but the IK-3 pilots claimed no victories.

A joint claim was made during the third German attack at 14:00, a twin-engined bomber by Gogić and another pilot from the 162nd Fighter Squadron.

[14] It became difficult to continue activity from the 51st Fighter Group airfield at Zemun due to air attacks, so on 8 April the remaining IK-3s and Bf 109Es flew to an auxiliary airfield at Veliki Radinci, 50 km (31 mi) north-west of Belgrade, where the surviving aircraft of the 6th Fighter Regiment were concentrated.

Poor weather made operations impossible until 11 April, when Semiz shot down a Bf 110 that had strafed the airfield.

[14] According to aviation writers Dragan Savić and Boris Ciglić, one serviceable IK-3 was captured by the Germans in April 1941 and it was joined by another by the end of June.

In late June, while the German guards were distracted listening to news of the invasion of the Soviet Union, local communists, including former VVKJ mechanics, moved the fence.

[16] "The IK-3s put up a valiant resistance against the Luftwaffe," wrote William Green, "scoring a number of 'kills' before they were finally destroyed in combat.