With the A320 unveiled to the world, Airbus refocussed on the widebody market with the medium- to long-range A330 twin-engine and longer-range A340 four-engine aircraft. These were launched jointly in 1987, just four months after the A320’s first flight. It marked another Airbus milestone: the world’s first combined aircraft programme, with both models sharing the same basic fuselage, wing and tail.
The A330 and A340 offered extensive cockpit system commonality, not only between themselves, but with the A320 Family as well. This made possible the Airbus concepts of cross-crew qualification – pilots trained to fly one Airbus aircraft could qualify to operate another with relatively little extra training – and mixed fleet flying, in which airlines are able to switch their Airbus aircraft and pilot crews to better match capacity to demand.
Source : Airbus Website