Qatar is the largest all Airbus operator in the Middle East.
The country has also expressed interest in Airbus parent group European Aeronautic Defence and Space Company (EADS), but there has been no specific discussion about the country becoming a shareholder.
Nicolas Sarkozy, the French president, present at the deal signing on Wednesday, vowed his government will seek new investors for EADS.
In another development, David Martinon, French presidential spokesman, said Qatar and EADS were to sign an agreement on June 3 on radar-based protection and safety measures for Qatari airspace.
"The A350XWB will equip Qatar Airways with the very latest generation and most modern fleet"
Akbar al Baker, CEO, Qatar Airways
Airbus has been losing customers for mid-sized jets to US rival Boeing and was forced last year to launch a costly redesign of the planned A350 to compete with Boeing's 777 and 787 Dreamliner, due to enter service in 2008.
The European aircraft maker has relied heavily on future demand for super jumbos to serve increasingly congested airports worldwide.
Qatar is a customer for the A380 super jumbo which has hit delivery delays.
But Airbus denies that this is linked to plans to cut thousands of jobs at its factory based in Toulouse in the south of France.
A tourism boom in the Middle East is contributing to the region's air traffic growth, powering the rapid expansion of airports and airlines.
The International Air Transport Association forecasts that international passenger air traffic in the Middle East will expand by an average annual growth of 6.9 per cent until 2010.