The car, built at the McLaren Technology Centre, sports the unique McLaren papaya orange, which echoes the colour chosen by Bruce McLaren for the team's first Formula 1 Grand Prix entry in 1966 and reflects the livery of the McLaren F1 team's MCL34.
This year's scheme for both series features a complementary blue that nods to the team's racing heritage, while the choice of #66 is to honour McLaren's first victory at the Brickyard, achieved with Mark Donohue's Penske-run M16 in 1972.
The #66 car is en route from the UK to the US ahead of the open test at the Indianapolis Motor Speedway on April 24. Meanwhile its sister test car will take to track at a closed test at the Texas Motor Speedway on April 9, in the hands of Fernando.
"The reveal of the #66 car and livery is an important moment for McLaren Racing's journey to the Indianapolis 500," said Zak Brown. "It represents the beginning of the next phase of our programme, and reflects a tremendous amount of hard work by our specialist Indy 500 team at the McLaren Technology Centre. We are all looking forward to seeing the #66 out on track for the first time on April 24."
"I'm excited we are finally able to reveal to the world the #66 car which I'll be racing with McLaren at the Indy 500 in May," added Fernando Alonso. "The team at the McLaren factory has worked very hard to build this car ready for our return to the iconic Brickyard and I think it looks fantastic in the 2019 McLaren Racing colours.
"My hopes for the race remain the same, to win and achieve the Triple Crown, and I'm looking forward to meeting the fantastic US fans who made me feel so welcome first time around."