Now entering service, the Hawker 900XP business jet is the latest variant of a line of business jets that have been in continuous production since 1962, longer than any other civilian jet. Only the Boeing 737 comes close, having been in production since 1968.
The original production version, the Hawker-Siddeley HS-125, was built and designed with the airframe failures of the De Havilland Comet in mind (the HS-125 design originated with De Havilland) had a robust structure that was even approved for operations from unpaved and sod runways. No airframe in this family of aircraft has ever suffered a failure as a result.
The first HS-125s were powered by Rolls-Royce Viper turbojets which were loud and not only fuel-thirsty, but also oil thirsty as the oil wasn't recovered from the bearings of the engine and one of the tasks of the copilot was to pour more oil into the turbojet before each flight. The first big leap in the design came in 1976 when Garrett TFE731 turbofan engines were added to the 125-700, making it, along with the Lear 35 and Falcon 10, one of the first turbofan applications in business jets.
The next leap that made the design a commercial success came in 1983 when the 125-800 rolled out with new longer span wing and a wrap around windshield to reduce drag.
There is an apocryphal story that when they were designing the cabin cross section for the HS-125, De Havilland engineers went to the London clubs frequented by executives that they expected to form the market for the aircraft and measured them to be sure the cabin was sized properly.
Source: Flying, October 2009. "Hawker 900XP- The newest version of a first generation business jet is the best one yet" by J. Mac McClellan, p43-45.