Airbus Group (AIR.PA) said on Sunday that its first U.S. factory, due to formally open on Monday, will be the most efficient in the world and will focus on building the largest version of the company’s single-aisle aircraft. The elegant, skylit factory on 116 acres here will be capable of making up to eight A320 family jetliners a month, though Airbus said it has no plans for that level of production. The factory is a potent challenge to rival Boeing Co (BA.N).