The world's second-largest plane maker plans an understated ceremony to mark the milestone, keeping the media at arm's length to safeguard the identity of its secret customer for the airplane dubbed the 747-8 Intercontinental.

Boeing (BA.N), which competes for orders with rival Airbus (EAD.PA), has taken 36 orders for the aircraft, which lists at $332.9 million. The airplane is more than 12 months behind its initial delivery schedule and some experts say the order book is puny.

"The 747-8 has been slow to take off, and the success of the aircraft is still questionable given so few orders," said Alex Hamilton, an aerospace analyst and managing director at EarlyBirdCapital.

Boeing had delayed the delivery to 2012 from the fourth quarter of 2011. The company blamed delays in flight testing and the time required to incorporate flight-test driven changes.

The first customer for the Intercontinental is an unidentified VIP. Boeing does not identify these customers, but past buyers of customized planes have been multimillionaires and heads of state.

The first airline set to receive the plane is Germany's Deutsche Lufthansa AG (LHAG.DE), which has ordered 20. Boeing has not set a delivery date for Lufthansa's first Intercontinental.

VIP customers for planes as large as the 747 often request extensive modifications such as bedrooms or bathrooms to accommodate the special needs of the primary passengers and their entourages. These modifications typically are done outside of Boeing, but the company must sign off on the changes.

The Intercontinental is the passenger version of the elongated, upgraded classic 747, which first flew more than 40 years ago. The 747 was the world's largest airplane until 2005, when Airbus unveiled its A380.

The aircraft incorporates some of the technology of the lightweight, carbon-composite 787 Dreamliner. It can seat 467 passengers, 51 more than the current version of the 747, but fewer than the competing 525-seat A380.

The freighter version of the 747-8 was first delivered in October. Orders for the freighter have been strained by an economic downturn that has dampened cargo markets.

Boeing made first delivery of its 787 Dreamliner last year after three years of delays. The 787 represents a bigger leap in technology than the 747-8.

(Reporting by Kyle Peterson in Chicago; Editing by Maureen Bavdek)

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