The plane-maker said it would improve the craft's aerodynamics by extending the tail cone and changing the way it integrates the engine with the wing.

Boeing (BA.N) last year unveiled plans to put new fuel-efficient engines in its existing 737 design. Boeing named the aircraft the MAX and said it would enter service in 2017.

"Based on design work and preliminary testing results, we have even more confidence in our ability to give our customers the fuel savings they need while minimizing the development risk on this program," said Michael Teal, chief project engineer and deputy program manager, 737 MAX program.

Boeing said it also would update its flight controls and strengthen landing gear, wings and the fuselage to accommodate the weight of the larger engines.

The MAX, which offers fuel savings of up to 12 percent over the current 737, will be powered by engines made by CFM International, a joint venture between General Electric Co (GE.N) and Safran (SAF.PA).

The airplane will compete with the Airbus A320neo, which also will feature new engines and offers fuel savings of 15 percent over the current A320.

Boeing has taken more than 1,000 orders and provisional commitments for the MAX since winning its first provisional order for the plane from AMR Corp's (AAMRQ.PK) American Airlines last year. Southwest Airlines (LUV.N) will be the first operator of the plane.

(Reporting By Kyle Peterson; editing by John Wallace)

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