The Aircraft Company was founded in 1911 to sell and maintain Farman aircraft at Hendon. It was later renamed The Aircraft Manufacturing Company. Its founder, George Holt Thomas, was a business man without engineering knowledge. He first met de Havilland at Farnborough when the latter was becoming dissatisfied with his job. Agreement was soon reached that Holt Thomas’ company should start manufacturing de Havilland-designed aircraft. In 1914 Geoffrey moved to Edgware and started work at the Airco factory at the Hyde, Hendon.

When World War One began in 1914 de Havilland was a Lieutenant in the Royal Flying Corps Reserve. He was called up but was fit only for home duties because of the effects of a flying accident. He served briefly at Montrose in Scotland, flying Bleriot monoplanes on anti-submarine patrols from Aberdeen to the Firth of Forth. The War Office realised, fortunately, that he was of more use designing aeroplanes so he was recalled to London and promoted to Captain.

Once back at Airco de Havilland started designing aircraft. They began to be called de Havillands and to carry type numbers prefixed by DH. No one remembers how the DH numbers came into being, but they stuck and everyone used them.

Designs included single and two-seat fighters, single and twin-engined bombers. de Havilland received a royalty for each one produced by Airco. Such was the size of the orders, however, that many were manufactured by other companies and were even built in the United States, Canada and Spain.

One of the most famous DH types of World War One was not actually designed by de Havilland. Airco was busy with other projects when it was decided to fit the Liberty engine to the DH. 9 so the work was given to Westland Aircraft at Yeovil, Somerset. With assistance from John Johnson of the Airco drawing office they produced the DH.9a. The type became the standard RAF day bomber and remained in service until 1931.

The Armistice brought the cancellation of production contracts throughout the aircraft industry, but de Havilland had the foresight to look to civil aviation. A number of DH.4 and DH.9 aircraft were modified to carry passengers and freight. de Havilland also designed the DH.16 which was based on DH.9a components. These aircraft were supplied to the fledgling airline industry and also used to fly delegates to the peace conference at Versailles .

This amount of work, however, could not sustain a company of the size of Airco and eventually Holt Thomas sold it to Birmingham Small Arms (BSA). BSA only wanted the buildings, however, and were not interested in aircraft so de Havilland decided that his best course was to form his own company.