German low-cost airline Air Berlin said on Saturday it ordered 25 of Boeing Co.'s new 787 Dreamliner planes, in a deal worth $4 billion at list prices.

The deal, announced at a Boeing event in Seattle the day before the first 787 is unveiled, is the biggest single order for the lightweight, fuel-efficient plane from a European carrier.

Boeing said it now has 677 firm orders for 787s. The first of the planes is set to roll out of Boeing's Everett, Washington, plant on Sunday, and is set for its first test flight in August or September and first delivery in May next year.

Air Berlin, based in the German capital, is Europe's third largest low-fare carrier. The airline is looking to add long-haul destinations to its route network with the addition of the mid-sized, long-range 787s, which can easily reach Asia and the United States from Germany.

In addition to the 25 firm orders, Air Berlin said it had taken options to buy a further 10 787s and had agreed purchase rights -- the loosest form of commitment in the process of buying an airplane -- on another 15.

Air Berlin did not announce an engine supplier for its 787s.