Question: In which county are the headquarters of this company located that developed one of the world's first commercial jet airliners built by Alan Samuel Butler?
Sentence (from "Alan Samuel Butler"): From 1923 Butler was chairman of De Havilland Aircraft Company —which he financed— until 1950, a year when De Havilland employed 20,000 people and was building Comet, the world's first commercial jet airliner.
Relevant evidence: Yes