The Solar Impulse team says the flights to Belgium and France are a big challenge because the plane will need to navigate across international air traffic networks.
The single-seater prototype with the wingspan of a Boeing 777 made its 2009 maiden flight in Switzerland and further tests have all taken place there.
Pilot Andre Borschberg said the success of the first cross-border flights depends on the team receiving authorization from national authorities.
The team says it will ready the plane from May 2 and take off to Brussels when weather conditions are right.
The plane is expected at the 49th International Paris Air Show from June 20-26.