The Stirling was a World War II heavy bomber built by Short Brothers. It was the first British design to use four engines and turned out to be inferior to its rivals. In the end it would be relegated to second-line duties, while four-engine conversions of earlier twin-engine designs took over its role. Through the 1930's the Royal Air Force was interested primarily in twin engine bombers. These designs put limited demands on engine production and maintenance, both of which were already stretched with the introduction of so many new types into service. However, the limitations in terms of power were so serious that they invested heavily in development of huge engines in the 2,000 horsepower (1500 kW) class in order to improve performance.