United Airlines finalizes order for 25 Boeing 787s

United Airlines said Thursday it finalized an order for 25 Boeing 787 jetliners.

The deal with Boeing, announced in December, also includes the option to buy 50 more 787s. The order is valued at $4.2 billion at average list prices, although airlines usually negotiate lower prices for the planes they buy.

United expects to take delivery of the 787s at the same time it will begin to retire its Boeing 747s and 767s operating on international routes.

United said the 787, currently in test flights, will provide greater fuel efficiency and range, allowing it to add flights to new cities.

The Chicago-based airline first announced in December it intended to buy 50 new planes, half Boeing 787s and half Airbus 350s. No planes will be delivered until 2016, and United has extensive rights to defer the orders.

Shares of Boeing, based in Seattle, fell $1.23, or about 2 percent, to $62.26 in midday trading. Shares of United's parent company UAL Corp. lost 22 cents at $15.45.