The de Havilland DH-106 Comet was introduced in 1951. The Comet was the first commercial transonic passenger jet, and also pioneered the use of new aluminum alloys in the airframe. Previous aircraft, mostly military designs from the Second World War, had used steel. The Comet's design made use of aluminum's greater strength-to-weight ratio.
Seven models of the Comet were produced, starting with the Mk 1 in 1951, through the 4C in 1959. All were 4-engine aircraft with cruising speeds from 450 to 830 MPH and cruising altitudes in the general neighborhood of 40000 feet (plus or minus a few thousand). Range varied from 1500 to 4300 miles.
In 1954 two Comets disintegrated mid-flight for no apparent reason, killing 56 people. Investigation of these crashes eventually showed that they were due to metal fatigue (or structural fatigue), a previously little-known phenomenon to which aluminum is especially susceptible. Later research developed less failure-prone alloys, as well as design techniques to avoid structural fatigue, but the reputation of the Comet (and arguably the fortunes of the de Havilland company) never fully recovered.