Boeing Co is considering forming a separate company to build its proposed 7E7 Jet and may offer equity stakes to partners, said Alan Mulally, chief executive of the firm’s commercial aeroplane unit. Mulally reaffirmed Boeing’s estimate of demand for the fuel-efficient new plane at about 2,500 to 3,000 units, and said Japanese firms were likely to take a share of over 20 percent in the programme. The 200-to 250-seat 7E7, which Boeing has dubbed the 7E7 Dreamliner, is scheduled to fly in 2008. By the first half of next year, Boeing hopes to ‘‘come to a really fundamental agreement with airlines’’ over the aircraft’s specific requirements, Mulally said. ‘‘I wouldn’t rule it out a separate company with equity shares,’’ he said.