It is the only big commercial aircraft made mostly of carbon-fibre rather than aluminium and is billed as the most environmentally friendly ever built.
Boeing says the 787 is much more fuel efficient than its competitors and produces 20% less CO2.
Boeing says it already has more than 600 orders. The first test flight is expected in August or September.
Sunday was chosen for Dreamliner's premiere as it is 8 July, or, in the American date style, 07/08/07.
Compare Boeing and Airbus planes
An audience of thousands were in attendance in Seattle as the aircraft was presented to the media.
"Our journey began some six years ago when we knew we were on the cusp of delivering valuable new technologies," Mike Bair, who heads the 787 programme, told the crowd.
