The Jenny generally used for primary flight training, but some were equipped with machine guns and bomb racks for advanced training. The Curtiss JN-4 is possibly North America's most famous World War I aircraft. It was a twin-seat (student in front of instructor) dual-control biplane. Its tractor propeller and maneuverability made it ideal for initial pilot training and was widely used during World War I to train beginning pilots. After World War I, thousands were sold on the civilian market for very low price. The "Barnstormer" airplanes of the 1920s attracted many future World War II air force leaders to the thrills of flying. Powered by a Curtiss OX-5, 90 horsepower (hp) engine it could reach 75 MPH and stay aloft for 2.5 hours.