Boeing, which most recently targeted first delivery in the third quarter, said in a statement it planned to deliver the first plane between August and September.
The 787 is a light-weight, carbon-composite aircraft whose sales pitch offers fuel savings for airlines and more comfort and less jet lag for passengers, with its purer and more humid air pressurised to a lower altitude than ordinary jets.
After nearly three years of production delays, the plane is being prepared for the first commercial operations.
Using the second flight test aircraft code-named ZA002, Boeing said it planned to carry out test flights in Japan together with ANA to help prepare for real operations.
The in-service test flights will take place in the week of July 4. Boeing expects to fly the Dreamliner between Haneda Airport in Tokyo and airports in Osaka, Okayama and Hiroshima.
The Dreamliner is the highest-profile project for Boeing Commercial Airplanes.
The aircraft has captured the imagination of the aviation world, but problems with the extensive supply chain, as well as labour disputes, have caused a string of delays.
ANA has said it is disappointed about the delays which are are expected to have resulted in costly penalties for Boeing.
Boeing is the world's second-largest commercial planemaker, behind Europe's Airbus which is developing its own mid-sized airliner to compete with the Dreamliner, the A350.
Boeing has taken 835 orders for the plane from 56 customers. (Additional reporting by Tim Hepher; Editing by Dan Lalor and Jon Loades-Carter)