McLaren described the new car as "innovative".
The Woking-based team finished in ninth place in the constructors' championship last year after a season full of reliability problems with the Honda engine.
The Japanese manufacturer returned to the sport in 2015 after having left following the 2008 season.
But its power unit proved uncompetitive and unreliable and McLaren managed a fourth place in Hungary as its best result.
Now the team is optimistic that several changes to the Honda engine will help turn its fortunes around in 2016.
The new car, pictures of which were leaked on the Internet before the scheduled launch, continues the "size zero" philosophy it opted for last year.
The MP4-31 also retains the very short nose ran last season.
"As we embark on the second year of our renewed McLaren-Honda partnership, all of us remain united in our purpose," said Ron Dennis.
"That purpose is to develop our team towards our shared ambition: to win.
"We'll make no predictions as to when those wins will come, but I can say without fear of contradiction that every member of our team has worked with truly relentless dedication over the past few months.
"The result is that MP4-31's developmental trajectory has been usefully steepened over the winter, and I'm consequently very proud of our team's efforts."
McLaren's new car will kick off its testing programme tomorrow at Barcelona in the hands of Jenson Button.
This article by Pablo Elizalde originally appeared on Motorsport.com, the world's leader in auto racing news, photos and video.