The Mercedes D.II was a German aircraft engine developed during World War I. It was a six-cylinder, water-cooled inline engine that produced around 150 horsepower. This engine was primarily used in various military aircraft, contributing to the performance of several fighter and reconnaissance planes. The D.II engine was notable for its reliability and efficiency, making it a popular choice among German aviators. It powered aircraft such as the Albatros D.II and the Fokker D.VI, enhancing their speed and maneuverability in combat. The D.II played a significant role in the evolution of aviation technology during the war.