The MB-3A contract was the government's largest order for new airplanes since the end of World War I. The MB-3, a conventional wood-and-wire fabric-covered fighter, was derived from the French Spad of World War I. The Boeing version differed from the original Thomas Morse design with the engine radiators situated on the sides of the cockpit rather than on top of the upper wings. Moreover, the machine guns were covered with an aerodynamic fairing. Some MB-3As had four-blade propellers and others two blade propellers. The MB-3A contract established Boeing as a reliable manufacturer of fighter aircraft. Following the MB-3A design, fighters became the main line of business for Boeing for a dozen years.