AIRSHOW-Dubai Airshow to open with Gulf spending spree
* Boeing seen launching 777X with around 250 orders, 4 buyers
* Airbus fights back to avoid rare air show defeat
* Boeing says union vote on 777X assembly won't deter launch
* Emirates airline seen in talks to buy 20-25 A380s -source
* UK's Cameron pays lightening visit on eve of show
By Tim Hepher and Praveen Menon
DUBAI, Nov 17 (Reuters) - Gulf airlines are set to place orders worth over $100 billion as the Dubai Airshow gets under way on Sunday, industry sources said, led by record investment in a new Boeing jet.
The birth of a revamped successor to Boeing's 777 jet, with some 250 orders booked or in the pipeline from four airlines, marks a new step in the growth of Gulf airlines and a fierce contest between Boeing and Airbus for their business.
Boeing is expected to launch the 777X jet with orders from the top three Gulf carriers -- Dubai's Emirates, Abu Dhabi's Etihad Airways and Qatar Airways -- and Germany's Lufthansa which has already tentatively committed to buy 34 of the planes. Continued...