Development stalled after a flight test by Lützow Holm, commander of the Karljohansvern base. After a rough landing, following drastic and hazardous flying, he demanded that the design be revised to strengthen it before entering production. The construction yard opposed this, pointing out that the design was already 10% stronger than had been specified by the Navy. While the dispute continued, the M.F.12 was grounded at Karljohansvern. In June 1939, shortly before the M.F.12 took to the air for the first time, the Norwegian Ministry of Defence set down a joint military and civilian commission to decide on a new common trainer for both the Royal Norwegian Navy Air Service, its Army cousin; the Norwegian Army Air Service, and the civilian air schools. Further work on the M.F.12 was put on hold while the commission worked on its report. Before any conclusion could be made, the Germans invaded Norway on 9 April 1940.