Swiss aviators Bertrand Piccard and Andre Borschberg have been taking turns piloting the plane, which has more than 17,000 solar cells built into wings whose span exceeds that of a Boeing 747, with Piccard at the controls for the transatlantic flight.
The plane's slow cruising speed, similar to that of a car, has required both men to take up meditation and hypnosis as part of training to stay alert for long periods.
Solar Impulse 2 is due to land sometime on Thursday in Spain or France, with the precise location to be determined later depending on weather conditions, said Elizabeth Banta, a spokeswoman for the project team.
The carbon-fibre, propeller-driven plane has four solar-powered engines and four batteries to store surplus energy. It weighs the same as a family car and can climb to 28,000 feet (8,500 m).
The team behind Solar Impulse - part of a campaign to build support for clean energy technologies - hopes to complete the circumnavigation in Abu Dhabi, where the journey began in March 2015.
Piccard and Borschberg completed a multi-flight crossing of the United States with an earlier version of the plane in 2013.
Borschberg set an endurance record for the longest non-stop solo flight last July in a 118-hour trans-Pacific crossing from Japan to Hawaii.
Coolest gadgets for flights
Coolest gadgets for flights
It may be one of the most anti-social in-flight accessories around, but the B-tourist is ideal for eating, sleeping and reading while pretending no-one on the plane exists. Made from stretchy fabric, it loops around the seat backs and features plastic rings on the sides to adjust the width of your privacy. The B-tourist is yet to hit the market.
Designed to keep you extra warm and comfortable on long-haul flights, the Duvet Suit comes in a carry bag that is condensed and can be used as a pillow and detachable hood. The suit is a two-piece outfit that can be worn as separates if you get too warm on the plane.
Having trouble getting comfy in your seat? The Ostrich Pillow was invented for power naps and allows you to comfortably rest your head on any surface (even the plane window), while blocking out visual and aural distractions. There's just one catch - it doesn't look good and you'll certainly attract a few stares with this quirky pillow that makes you appear to be wearing a padded balaclava.
It might not be the most fashionable jacket, but if you want to stay cool on a plane, the Kuchofuku Air Conditioned Jacket may be the answer. The jacket features two cooling fans on its back which waft a refreshing breeze around the wearer. The fans are powered by AA batteries or can be plugged into a computer using a USB cable.
One of the most controversial travel gadgets to date, the Knee Defender has caused quite a stir and has even diverted flights. The gadget works by helping you stop reclining seats on planes. It clips onto your meal tray and prevents the person in front from reclining. The plastic gadget, which has been criticized as being a passive aggressive product, even comes with a 'Courtesy Card' which you can present to the passenger in front asking them not to recline their seat. The creators of the product say that if this is not effective, the Knee Defender acts as your 'Plan B'.
Need help falling asleep on a plane? SleepPhones provide restful, audio-assisted relaxation without bulky headphones or uncomfortable earbuds. The wearable technology works as pyjamas for your ears, using smooth fleece material and helping you fall asleep on a flight while reducing ambient noise, conversations and snoring.
Conscious of plane germs? The trayGUARD is a tray table cover that provides an instant hygienic surface for flyers. The inventors say it is food contact safe and incorporates a chemical-free antimicrobial technology that is effective for the duration of your trip. Just slip it over your tray and you won't have to worry about catching flu germs. There is even a kids' version which comes with crayons for children to decorate their trayGUARD.
Does dozing on a flight leave you with neck pain? For those not getting enough support from U-shaped neck pillows, the NapAnywhere head support could be the answer. The pillow was developed by physician Dr. Shamaiengar to make travelling a more comfortable experience and is a minimalist device providing head support while in a seated position. The top part of the NapAnywhere features a shelf which sits below the chin and back of the head with a design to cradle your head.
Aero-Tray allows you to make the most of your space on a flight with its two-tier system which can separate food and drinks from your electronic items. It features an extendable pull-out shelf and cup support while counteracting the angle of a reclined seat in front, elevating your computer screen to relax neck, wrists and eye strain.
Part pillowcase, part hoodie, part iPhone case, the HoodiePillow promises ultimate Cocoonification™ to help you tuck away your stresses and relax on a flight. The travel pillowcase offers warmth, quiet and a healthy sleep environment while keeping the light out to give you uninterrupted shut-eye on a plane. It fits standard bed pillows and is made from premium sweatshirt material, which features two drawstrings for you to adjust it to your head size.