United Airlines is looking at the latest upgraded A321 model from Airbus as well as concepts being touted by Boeing as potential replacements for its Boeing 757 fleet, but does not see a purchase in the near future, a senior executive said on Tuesday. The airline industry is increasingly debating the future of a potentially promising niche between popular medium-haul aircraft, like Boeing’s 737 or the Airbus A320 family, and the smallest wide-body or twin-aisle jets. The gap includes future replacements for the Boeing 757, which halted production a decade ago.