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The planes are "valued at nearly $1.5 billion at list prices," the US manufacturer said in a statement.
PIA "has been renewing its long-haul fleet to accommodate increased demand for air travel as well as to introduce new routes," Boeing said.
PIA managing director Nadeem Yousufzai lauded the 777-300ER's "excellent operating economics, long range capability and reliability."
Boeing said it had logged 200 net orders for 777s in 2011, by far the best year for the family of aircraft, in service since 1995.