Solar Impulse's HB-SIA solar-powered airplane was runway tested last week. The prototype aircraft is made of lightweight materials, weighing only 3,500 pounds and it has a wingspan of 210 feet. It is powered entirely by the 11,000 solar cells covering its wings. It is intended to fly at only 28 miles per hour to keep energy consumption low. It will store solar energy for night flight
Founder of Solar Impulse, Bertrand Piccard, a former astronaut and the first man to circle the world nonstop in a balloon, hopes to perform the same feet in a solar-powered plane derived from on the HB-SIA design. Solar Impulse aims to test the prototype in flight next year and to achieve a 36-hour flight without fuel shortly after that. Results from these tests will be used to build a solar-powered plane to will attempt a transcontinental flight sometime after 2012.
Take a look at this nicely produced video of Solar Impulse's HB-SIA runway test.
(Solar Impulse HB-SIA runway test video)
As far as I know, actual solar-powered aircraft didn't appear until the 1970's, in very small UAV form. I don't know who first thought of them, but I wasn't surprised to see a reference in John W. Campbell's 1930 novel The Black Star Passes:
"Dad, I believe that you have been trying to develop a successful solar engine. One that could be placed in the wings of a plane to generate power from the light falling on that surface..."
(Read more about Campbell's solar-powered airplane)