Maintaining their skills in designing trainer aircraft, the company built a more robust biplane called the Avro Tutor in the 1930s which the Royal Air Force (RAF) also bought in quantity. A twin piston-engined airliner called the Anson followed but as tensions rose again in Europe the firm's emphasis returned to combat aircraft. The Avro Manchester, Lancaster, and Lincoln were particularly famous Avro designs. Over 7,000 Lancasters were built and their bombing capabilities led to their use in the famous "Dam Busters" raid. Of the total, nearly half were built at Avro's Woodford (Stockport) and Chadderton (Oldham) sites, with some 700 Lancasters built at the Avro "shadow" factory next to Leeds Bradford Airport (formerly Yeadon Aerodrome), northwest Leeds. This factory employed 17,500 workers at a time when the population of Yeadon was just 10,000. It was the largest building in Europe at the time, at , and its roof was disguised by the addition of fields and hedges to hide it from enemy planes. The old taxiway from the factory to the runway is still evident.