Michael Mecham Seattle, Robert Wall London,Maxim Pyadushkin Moscow and Bradley Perrett Beijing

For now, Airbus and Boeing are sparring in PowerPoint over whether the A320NEO or the 737 MAX is the more fuel-efficient jet. But customers will soon have more concrete data on which to make their own evaluation of commercial aviation's newest single-aisle products.

View an interactive comparison of new features on the A320NEO and 737 MAX.

In the next few months, Airbus plans to lock in the detailed design elements of the A320NEO that define exactly how the wing and wingbox need to be stiffened to handle the airplane's larger and heavier engines, as well as its winglets. In addition, the first metal will be cut on the A320NEO this month, says Tom Williams, executive vice president for programs.

Final assembly of the first aircraft is planned in the second quarter 2014. Although the NEO will eventually roll off all the existing Airbus narrowbody final assembly lines—in Hamburg, Toulouse and Tianjin, China—the French facility will be the first in production. A decision on the new Mobile, Ala., line has not been made. First flight is planned in the last quarter of 2014.

At Boeing, which put MAX into the reengining fray a year after Airbus, aerodynamic tradeoffs have been completed on a design that centers on a regauged fuselage, strengthened wing with a new winglet, beefed-up landing gear and aft-body aerodynamic improvements, says Vice President and 737 General Manager Beverly Wyse. Architectural tradeoffs will be completed by the end of the third quarter.

The detailed configuration of the MAX as the reengined upgrade of the 737 Next Generation series is due by mid-2013. MAX's design is to be set in 2014, the first assembly will follow in 2015 and first flight in 2016. Certification and delivery are set for the first half of 2017. The initial MAX delivery is a 737-8 to Southwest Airlines.

Boeing, meanwhile, has identified space for a third production line at its Renton factory south of Seattle to accommodate MAX's production and is rebuilding parts of the factory to accommodate higher production rates. It expects 737 Next Generation orders to taper off substantially by 2018 in favor of the fuel efficiency that MAX offers.