By Tim Hepher PARIS (Reuters) -Airbus is poised to stage the maiden flight of its A321XLR jetliner – a major milestone in the race to improve the range of narrowbody jets – but it is struggling to pin down key certification and delivery dates, industry sources said. Airbus says the A321XLR can fly 4,700 nautical miles (8,700 km), 15% further than its most capable current narrowbody jet, allowing airlines to serve routes like New York-Rome, Houston-Santiago or Tokyo-Sydney without the cost of a big twin-aisle. The extra range is made possible by the addition of a new rear-centre fuel tank mould…