In the USA, the F 13s sold there under the name JL 6 unfortunately attracted attention not only because of their robustness and outstanding flight performance, but also because of spectacular accidents.
In the period from May 1920 to February 1921, four mail cargo flights caused devastating fires.
Seven airmen and companions died.
The cause was the wearing influence of benzene fuel on rubber fuel lines.
In addition, studies have shown that benzene solidifies at extreme sub-zero temperatures (such as flights in colder regions or at high altitudes).
The rubber lines were blocked and cracked because of the vibration.
When returning to warmer air layers, the benzene dripped from the cracks and led to fires.
These complications ended when gasoline was used as fuel.
Nevertheless, the sale of the remaining eight aircraft, which Larsen still had at the beginning of 1921, was very difficult because of these accidents.
Therefore, Larsen looked for new ways of exploiting, had at least one JL 6 converted for military purposes and gave this version the designation JL 12.
Machine guns with ammunition drums were installed in the hold (according to different sources between 12 and 28) in certain bundling and in rows one behind the other.
The barrels protruded partly slightly forward and partly backwards inclined from the aircraft floor, and they could be put into action by the gunner, who sat next to the pilot in the cockpit, by means of a central release during the target flight.
Accordingly, the JL 12 was a flying machine gun fire nest and should be used to fight enemy infantry as well as vehicle columns.