All Airbus single deck widebodies can trace their history back to A300B2, the 1st production aircraft of Airbus.
Airbus planned to develop 3 more widebody aircrafts immediately after the launch of A300B2 in early 1970's, the shorter variant of A300 (which is A310-200), the stretched, longer range twin engined TA9 & quad engined ultra long range TA11. But the TA9 & TA11 plans were shelved until late 1980's, and they become A330 & A340 respectively.
What happens if Airbus develops TA11 early in 1970's? There would be many airlines of remote regions choosing it because of lower cost per trip & lower capacity compared to B747 and longer range. Airlines like Aer Lingus, Air New Zealand, LanChile, Aerolineas Argentinas would choose TA11 instead of B747 because their routes have too few passengers to support B747.
And TA9 would be very popular for many carriers because of much lower cost compared to DC-10 & L-1011 trijets of similar capacity.
The Airbus A300 has cross-section of 222", and interior width of 208". This is the best cross section width available, because this allows installation of standard LD-3 containers, rather than smaller LD-2 containers of Boeing B767. The A300 cross section, however, is not too wide so only 8 economy class seats of 18" can be installed per row, thus making every passengers no more than 1 seat from the aisle. Only 6 long haul business class seats per row can be installed, so there is no middle seats like that of B777 & B747. Not only this, there are 2 armsets between the 2 middle seats of economy class, so passengers can have more privacy & comfort.
A300 is the first aircraft featuring articulated bins with lower side wider than the upper side. This not only greatly increases overhead luggage space, but also less nuisance on door opening.
The use of articulated bins instead of drop down bins not only eliminates the danger of luggage fall, but also ensures high height clearance of 1.9m below bins in both door close & door open conditions.
The centre bins can also be uninstalled for high centre ceiling of over 2.3m, much better than Boeing widebodies, which cannot have the centre drop down bins removed.
Such excellent cabin design are also used in A330 & A340, which are of same cross section width.
The A300 & the smaller A310 are very innovative aircrafts in 1970's to 1980's.
The A300B4, launched in late 1974 after A300B2, is the first ETOPS compliant aircraft in the world, thus allowing shorter & more versatile routing than twinjets without ETOPS. It is also the 1st aircraft equipped with Cat IIIA autoland system.
In 1982, the A300FFCC (forward facing crew cockpit) was the first twin-aisle aircraft that could be piloted by a two-man crew, since all cockpit flight instruments were displayed in front of the flight crew.
In 1983, Airbus introduced the first advanced cathode ray tube (CRT) cockpit displays on the A310. The A310 also marked the introduction of composite materials in secondary structures and of electrical signalling for secondary controls..
In 1985, the A310-300 was the first commercial airliner to feature drag-reducing wingtip devices, which have since become an industry standard. It also marked the introduction of composites in primary structures and was the first sub-sonic civil airliner to embody trim tank/centre of gravity control.
Airbus finally launched A330-300, A340-200 & A340-300 in late 1987 under the same plan. They share the same wings, tainfins & fuselages, with enging pylons being the only different parts, thus greatly reducing maintenance costs. The A340-200 has range of 14800km, the longest range aircarft in the world until the A340-500. The A340 makes air service to remote regions more economical to run, and allows airlines to increase frequency of air routes served by B747.
They also use the same cockpits (except engine controls) very similar to smaller A320, thus flight crews flying A320 can fly A330 after just 8 working days of training instead of 25 days for aircrafts without commonality. A330 flight crews can fly A340 just after 3 days training, and 1 day training from A340 to A330.
These make airlines can operate both A330 & A340 with much lower cost, thus there are always best aircrafts for different routes, like A330 for short routes & A340 for long routes going via remote regions. Cathay Pacific is the 1st airline in the world to operate both A330 & A340 since 1996.
Because of this, many airlines are changing to full Airbus fleet, like Lufthansa, LTU of Germany, Swiss Air Lines, Austrian Airlines, South African Airways, BMI of UK, Virgin Atlantic, SAS, Air Canada, US Airways, Air Jamaica, LanChile, TAM of Brazil. Some airlines like Cyprus Airways, Middle East Airlines of Lebanon, Qatar Airways & Dragonair of HK are full Airbus fleet already.
Airbus launched larger A340-500 & A340-600 since late 1997, with -500 being the longest range aircraft of 15500km, and -600 the largest Airbus jet to day with capacity of 380 persons, thus making the -600 variant the ideal 1st generation 400-seat jets replacement.
Sadly Airbus did not launch A330-500 & A330-600 of same size as A340-500 & A340-600, which is to compete with B777, and provides best economy for short distance high passenger routes like intra-Asia routes with better comfort than B777. If Airbus launches the larger A330 then many Asian airlines running both A330 & A340, like Cathay Pacific, Singapore Airlines, Garuda Indonesia would prefer the larger A330 instead of B777.
Airbus is going to launch A330-200F, with a model shown in 2002 Farnborough Airshow. It is of 60-ton capacity & 10000km range, with 20% more capacity & twice range compared to A300-600F. This breaks Boeing monopoly on intercontinental freighters.
The sales of A300 & A310 is ever falling because of old 1970's design & hence poor economy. So Airbus is also considering smaller variants of A330. Hope this becomes true, so airlines can operate routes served by A300/A310 with lower costs.