Real World Information
The Stearman Model 75 is a biplane used as a military trainer aircraft, of which at least 10,626 were built in the United States during the 1930s and 1940s. Widely known as the Stearman, Boeing Stearman or Kaydet, it served as a primary trainer for the USAAF, the USN (as the NS & N2S), and with the RCAF as the Kaydet throughout World War II. After the conflict was over, thousands of surplus aircraft were sold on the civilian market. In the immediate postwar years they became popular as crop dusters, sports planes, and for aerobatic and wing walking use in airshows.
For more information see the Wikipedia Page
In the Game
For it's sole appearance in the series, the Stearman is given a red paint job, giving it a better contrast against the blue sea backgrounds of which most of the game is comprised of. It uses a similar weaponry which the P38 used in 1943: The Battle of Midway, including the ability to call forth natural phenomena to hit everything on screen.
In the PC Engine version of 1943 Kai, halfway through the game the Stearman gets an upgrade which adds rocket propellers to the ship and improves it's weapons.