 After protracted development including extensive wind tunnel testing of the design (especially of the T-tail which was tunnel-tested for 375 hours), the first prototype flew for the first time on October 27, 1972; a second prototype took to the air on December 15 the same year.Three production aircraft were also built in 1972 and delivered to the U.S. Army; these three were designated Model A100-1s by Beechcraft and were given the military designation RU-21J; the first of some 400 T-tail King Airs to be ordered by the U.S. armed forces.The 200 received civil certification in December 1973 and the first civil delivery took place in February 1974.