The solar-powered aeroplane, Solar Impulse, has completed a three-day flight over the Pacific Ocean.
It flew over San Francisco's Golden Gate Bridge on Saturday evening as it prepared to land in California.
The plane took off from Hawaii on Thursday, where it underwent repairs for the past eight months after its batteries were damaged during the flight from Japan.
This is the ninth leg of its attempt to fly round the world.
"I crossed the bridge. I am officially in America," said pilot Bertrand Piccard as he flew above San Francisco Bay.
Solar Impulse started the journey last March in Abu Dhabi. The trip has involved two different pilots flying separate legs.
Piccard will land the plane later on Saturday evening at Moffett Airfield, located in Mountain View in Silicon Valley.
The landing is being delayed until winds drop.
Solar Impulse gets all its energy from the sun - through the 17,000 photovoltaic cells that cover the top surfaces of the craft.
These power propellers during the day, but also charge batteries that the vehicle's motors can then call on during the night.
The distance on this leg was 4,000km or 2,200 nautical miles.
Starting in Abu Dhabi, UAE, in March, Solar Impulse crossed Oman, India, Myanmar, and China.
It then flew to Japan, before undertaking a 8,924km passage to Hawaii.
That five-day, five-night crossing set a record for the longest ever non-stop solo aeroplane journey.
But the vehicle's batteries overheated during the trip, forcing the project to stop on the Pacific archipelago while repairs were conducted.
A further 20m euros (£16m; $23m) had to be raised from supporters during the winter to keep the project going for another year.
Piccard shares flying duties with his business partner, Andre Borschberg.
It was Borschberg who flew into Kalaeloa last July, and he will next take the controls on the next leg across the US mainland.
The pair's intention is to reach New York by the start of June, to begin preparations for an Atlantic crossing.
Assuming this is completed successfully, it should then be a relatively straightforward run back to the "finish line" in Abu Dhabi.
Piccard and Borschberg have been working on the Solar Impulse project for more than a decade.
They first trialled a smaller plane, taking it on a trans-America crossing in 2013.
The version of the vehicle they currently fly is considerably bigger.
Its wingspan is wider than a 747 jumbo jet, and, yet, it weighs only 2.3 tonnes.
Because the prop-driven craft moves so slowly, mission legs can take several days and nights of continuous flight.
This means Piccard and Borschberg - whoever is at the controls - have to stay alert for nearly all of the time they are airborne.
They are permitted only catnaps of up to 20 minutes - in the same way a single-handed, round-the-world yachtsman would catch small periods of sleep.
They also have to endure the physical discomfort of being confined in a cockpit that measures just 3.8 cubic metres in volume - not a lot bigger than a public telephone box.
But Borschberg says the experience so far has been exhilarating.
"An experimental plane is a living creation," he told BBC News.
"Each flight you do brings new learning that you can use to improve the quality, reliability and performance of the aeroplane."