Under the leadership of Mr Kjos, aged 72, Norwegian Air developed from a small domestic airline into Europe's third biggest low-cost carrier.
It also broke into the transatlantic market with low fares.
However, the firm has struggled to make profits and has also been hit by the grounding of the Boeing 737 Max plane.
Norwegian has 18 of the aircraft and on Thursday said that it did not expect them to return to service until October, later than its previous estimate of August.
The airline said the grounding could cost as much as 700m kroner (£65m; $82m) this year, and could also undo its plan to return to profitability.
The 737 Max fleet of jets was grounded after two crashes, the first a Lion Air flight which crashed into the sea off Jakarta last year, and the second an Ethiopian Airlines' flight which crashed shortly after take off from Addis Ababa in March. In total 346 people were killed.