In the 1930s, the Royal Air Force (RAF) 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. Power limitations were so serious that the British invested in the development of huge engines in the 2,000 horsepower (1,500 kW) class to improve performance. During the late 1930s, none of these were ready for production. The United States and the Soviet Union were pursuing the development of bombers powered by arrangements of four smaller engines, the results of these projects proved to possess favourable characteristics such as excellent range and fair lifting capacity and in 1936, the RAF also decided to investigate the feasibility of the four-engined bomber.