Boeing Co flew its twice-delayed 747-8 Freighter for the first time on Monday, a significant milestone in the history of the legendary 747 family and a step that could bolster the credibility of the world's second-largest plane maker.

The new plane -- the biggest commercial jet Boeing has ever built -- completed a 3-1/2 hour flight around the Puget Sound area from an airfield north of Seattle, Washington, without any hitches.

Boeing twice delayed the first flight of the 747-8 last year, most recently moving a planned fourth-quarter flight to early 2010 and first delivery to the fourth quarter of 2010.

The 747-8 Freighter, about 18 feet longer than the 747-400, had been launched in November 2005 and was originally scheduled to start delivering in the fourth-quarter of 2009.

The company took a $1 billion charge related to the 747-8 in the third quarter of 2009 because of high production costs and tough market conditions.

Boeing, which has 108 orders for 747-8s -- 76 for its freighter model and 32 for the passenger model -- on its books at list prices between $293 million and $308 million, gets paid by customers at delivery.

The 747 family has been in the air since 1969 and is Boeing's biggest and most recognizable commercial plane.

The 747-8 uses new engine and wing designs, boasts greater fuel efficiency and lower operating costs than the Airbus A380, its closest rival, Boeing says. The Freighter model can carry 16 percent more cargo than the previous 747 model, while the Intercontinental passenger model can carry 51 more passengers.

The freighter's test flight comes on the heels of a successful test flight of the 787 Dreamliner in December.

Though not nearly as innovative or fuel-efficient as the revolutionary carbon-composite 787 Dreamliner, the 747-8 shares technology with the higher-profile plane.

Boeing's reputation has been bruised by two years of Dreamliner delays. That plane finally flew for the first time in December.

(Reporting by Kyle Peterson; Additional reporting by Bill Rigby; Editing by Tim Dobbyn and Steve Orlofsky)