In April 1939, shortly after the first Japanese Zero began test trials, modification drawings were already taking place on the zero in the Mitsubishi design room, to keep pace with the anticipated long air war in the Pacific. After the initial superiority of the Zero over allied aircraft began to wane, due to new U.S. types being introduced, Mitsubishi was hard pressed to maintain a superiority. The introduction of the Grumman F6F and Chance Vought F4U, provided a need for more firepower, armor protection and capability led the combined companies of Nakajima and Mitsubishi to produce the A6M5 series of aircraft.