He was commissioned a second lieutenant (on probation) in the Royal Flying Corps on 2 September 1912, was appointed a reserve officer in the RFC on 24 November and was confirmed in his rank on 25 December. In December 1913, de Havilland was appointed an inspector of aircraft for the Aeronautical Inspection Directorate. Unhappy at leaving design work, in May 1914 he was recruited to become the chief designer at Airco, in Hendon. He designed many aircraft for Airco, all designated by his initials, DH. Large numbers of de Havilland-designed aircraft were used during the First World War, flown by the Royal Flying Corps/Royal Air Force. De Havilland continued to serve in the RFC during the war. On 5 August 1914, he was promoted to lieutenant and appointed a flying officer in the RFC from the same date. He was briefly stationed in Montrose on the east coast of Scotland as an officer on war duty. Flying a Blériot, he was to protect British shipping from German U-boats. After a few weeks he was released from this duty and returned to Airco. However, he nominally remained in the service until the end of the war. On 30 April 1916, he was promoted to captain and appointed a flight commander. His employer, Airco, was bought in early 1920 by armaments group Birmingham Small Arms Company but discovering it was less than worthless BSA shut down Airco in July 1920. With the help of former Airco owner George Holt Thomas he formed de Havilland Aircraft Company employing some former colleagues. Pleased and impressed by the aircraft they built for him Alan Butler, thereafter company chairman, provided the capital to buy premises and then the airfield at Stag Lane Aerodrome, Edgware, where he and his colleagues designed and built a large number of aircraft, including the "Moth" family. One of his roles was as a test pilot for the company's aircraft. In 1928 the subsidiary company De Havilland Canada was created to build Moth aircraft for the training of Canadian airmen. After the Second World War it went on to design and produce a number of indigenous types, some of which proved highly successful. In 1933 the company moved to Hatfield Aerodrome in Hertfordshire. De Havilland was believed to have said "we could have had jets" prior to the start of the World War II although the company did not pursue this technology. In 1944 he bought out his friend and engine designer Frank Halfords's consultancy firm, forming the de Havilland Engine Company with Halford as head.