In the 1930s, the Royal Air Force (RAF) was primarily interested in twin-engine bombers. These designs put significant demands on engine production and maintenance, both of which were already stretched with the introduction of many new types of aircraft into service. Power limitations were so serious that the British invested heavily in the development of huge engines in the 2,000 horsepower (1,500 kW) class in an effort to improve performance. However, during the late 1930s, none of these engines was ready for production. Meanwhile, both the United States and the Soviet Union were developing bombers powered by arrangements of four smaller engines with favorable results, including excellent range and fair lifting capacity. Accordingly, in 1936, the RAF decided to investigate the feasibility of a four-engined bomber.