Kuwaiti lessor ALAFCO has firmed its tentative deal for 30 Airbus A320neo jets and added 20 more to the order.
It is complementing the firm deal for 50 of the twinjets with 30 options, to be finalised by the end of the year.
ALAFCO chief Ahmad Al Zabin signed the agreement during the Dubai air show.
The lessor has previously ordered 26 A320s, of which 25 have been delivered.
Airbus values the firm part of the deal at $4.6 billion at list prices.
ALAFCO sealed a tentative order for 30 A320neos at the Paris air show.
No engine selection for the aircraft was disclosed. The A320neo is available with the Pratt & Whitney PW1100G or the CFM International Leap-1A.
Al Zabin described the twinjet as an "absolute must-have" for the Kuwaiti company's fleet.
The lessor also has 18 Airbus A350-900s on order but has been cutting back on its Boeing 787 backlog with transfers of the orders to Saudi Arabian Airlines and Oman Air.
Al Zabin said that, at the time of the 787 orders in 2006-07, the company planned to be a "strategic partner" to Boeing in the area.
He said: "Whatever is to the benefit of Boeing and ALAFCO, we'll just do it."
Al Zabin said that ALAFCO was satisfied with its narrowbody and mid-sized portfolio and did not intend to move into the high-capacity sector. He ruled out the acquisition of types such as the Airbus A380.