* 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
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.
Gulf airlines have been expanding rapidly as they seek to take advantage of a strong geographical position connecting East and West.
A fifth, Hong Kong's Cathay Pacific, could join the programme later, industry experts said, while British Airways has expressed interest in the jet.
Boeing's top planemaking official pledged not to let a dispute with Seattle assembly workers over where the plane should be built interfere with its launch, which is expected to be announced on day one of the Nov. 17-21 show.
Boeing is looking for a home for the new jet after members of the International Association of Machinists rejected a proposed contract that would have seen Boeing commit to keeping the latest member of the 777 series near Seattle in exchange for restructured benefits.
The Middle East's top business event is seen as an opportunity to settle scores among rival Gulf airlines, who were seen as jostling for prime-time slots as the show started.
It is also a showdown between plane bosses who chase each other round the globe for new deals and have set up camp this week in the Gulf.
Airbus, which has a record of springing surprises at air shows, is keen to prevent a smooth lift-off for the 777X and is negotiating deals for all sizes of its jets including the A380.
"Airbus is desperate to blunt the impact of the 777X," said a senior industry source, speaking on condition of anonymity.
The event is a personal challenge for combative Airbus sales chief John Leahy, a New Yorker who is facing a rare defeat at an air show in his 20th year in the job.
Leahy is especially keen to boost the popularity of the A380 superjumbo, which has found no buyers this year and faces a cut in production unless empty 2015 production slots can be filled.
All eyes will be on Emirates which has frequently said it would add another 30 aircraft to its existing order for 90 superjumbos once it resolves capacity constraints in its hub.
An industry source said Emirates was in talks to order another 20-25 A380s but that this could rise.
"With Leahy in town who knows," the industry official said.
Bloomberg News reported the Emirates airline was talking to Airbus about a purchase of as many as 50 A380s, a record for the jet.
Boeing and Airbus declined comment, while Emirates was not immediately available for comment.
Boeing's 777X comes in two models including what will be the world's longest-distance passenger jet, a 350-seat model to be known as the 777-8 once the aircraft has been launched.
The larger 777-9 edition, carrying 406 people, will be the main version and be delivered starting 2020.
Together, the modernized planes call for development of carbon-fibre wings that fold at the tips to fit in the same parking spaces and new engines from General Electric.
Airbus says Boeing has packed in passengers densely to make the revamped aircraft's economics work against its own all-new 350-seat model, the A350-1000, due to enter service in 2017.
It has launched a campaign for a minimum standard seat width of 18 inches on long trips, aiming to draw attention to what it says will be the 777's narrower seats.
Some airlines have told Airbus that this is their decision and Boeing says many Airbus jets have similar seats.
A group representing U.S. airline pilots meanwhile warned on Saturday that the sale of hundreds of planes to Gulf carriers that compete with U.S. carriers would have "serious consequences for the U.S. economy and U.S. airline workers." This would create stiff competition for the U.S. airline industry.
In a reminder of potential defence deals also at stake, UK Prime Minister David Cameron visited the site of the show on his way back to London from a Commonwealth summit in Sri Lanka.
The United Arab Emirates is weighing whether to buy French Rafale or UK-backed Eurofighter jets.
"Get out there and win," Cameron told heads of UK aerospace and defence firms preparing for the show, organizers said.
(Additional reporting by Andrea Shalal-Esa, Nadia Salem, Mahmoud Habboush, Mirna Sleiman; Editing by Diane Craft)