The huge plane took off from Everett's Paine Field shortly after noon and returned to Paine at 4:18 p.m. PST after an approximately 3 1/2-hour flight.
A crowd Boeing estimated at more than 5,000 employees, customers, suppliers and other airplane fans gathered to watch the plane take to the air. The flight came just one day short of the 41st anniversary of the first flight of the original 747 model.
At 250 feet long - more than twice the length of the Wright Brothers' first flight - the plane is about 18 feet longer than the existing 747-400 jumbo jet. The company conducted taxi tests on the freighter Saturday, with the aircraft performing well, Boeing said.
"The airplane performed as expected and handled just like a 747-400," said 747 Chief Pilot Mark Feuerstein, who was joined on the flight deck by Capt. Tom Imrich.
Monday's flight around western Washington began a testing program that will involve more than 1,600 flight hours. As pilots checked basic handling and engine performance, the plane reached a cruising altitude of 17,000 feet and speeds as high as 264 miles per hour.
Boeing also is developing a passenger version of the plane. It lists 76 orders for the freighter and 32 for the 747-8 passenger jet, with the vast majority from international customers.
The company says the jets will be much quieter, more fuel efficient and have lower emissions than current 747-400 models.
Boeing launched the freighter program on Nov. 14, 2005, with firm orders for 10 planes from Cargolux of Luxembourg and eight from Nippon Cargo Airlines of Japan. The jet has a list price of more than $301 million, though airlines commonly negotiate discounts.
After completing the test program, the first freighter will be refitted and delivered to Cargolux.
The freighter version is to enter service late this year. The first delivery was to have been in late 2009 and the first passenger version in late 2010, but Boeing pushed back the dates due to design changes, limited engineering resources and an eight-week strike that shut down factories.
Boeing's European rival Airbus had planned a freighter version of the Airbus A380, the world's largest passenger jet. However, that program was put on hold in 2007 after FedEx Corp. and UPS Inc. canceled their orders, leaving Airbus with an empty order book for the cargo plane. The A380 airliner made its first flight in April 2005 and went into service in October 2007.
The 747-8 freighter and passenger jets are much smaller than the A380, which Boeing says is an advantage. It says its planes will cost less to operate and will be able to serve more markets.
The 747-8 passenger version will carry up to 467 people in three classes, with a range of just under 7,000 miles. Boeing says assembly of that plane is to begin around mid-2010, with the first delivery in the fourth quarter of 2011.
Boeing said in October that it was recording a $1 billion charge because of delays in producing the new freighter.
The company also is conducting the flight test program for its new 787 passenger jet, which made its first flight in December, more than two years behind schedule.