opean discount airline Ryanair placed an order worth up to $15.6 billionwith Boeing Commercial Aircraft, planning to take delivery of 175 Next-Generation 737-800 jets for its scheduled expansion.

The purchase reportedly will be Boeing’s largest European order to date, and the new jets will help Dublin-based Ryanair add flights as it expands its service into new markets. Ryanair intends to introduce service to Spain, Hungary, Scandinavia, and other markets that are losing service because rival airlines have downsized or gone out of business.

Reportedly, about 75 of the new jets will replace older units, but the majority will represent increased carrying capacity as Ryanair increases its fleet of 737-800s to about 400.

Boeing’s 737 is frequently described as the best-selling plane of all time, a twin-engine narrow-body jet, and the 737-800 is a stretched version of the “Next Generation” model, seating 162 to 189 passengers.

"This agreement is an amazing testament to the value that the Next-Generation 737 brings to Ryanair," stated Boeing Commercial Airplanes president & CEO Ray Conner. "We are pleased that the Next-Generation 737, as the most efficient, most reliable large single-aisle airplane flying today, has been and will continue to be the cornerstone of the Ryanair fleet."

In addition, Ryanair may expand its order to include up to 200 of Boeing’s forthcoming 737 MAX. That would increase the order value to more than $20 billion.

The revised version of the 737 will have a commercial debut in 2017, following a design program that aims to make the jets lighter and more fuel-efficient.

“Today’s order is very much an interim order and a forerunner for what I very much hope will be an order for Max aircraft once our teams finish their study,” Ryanair CEO Michael O’Leary said at a press conference. “We both hope the joint team we’ve established will report back and come up with a deal for a Max order sometime before the end of this calendar year.”