See the coolest cabin features on Boeing's long-awaited 777X

It's seven years late and has racked up over $15 billion in charges, but Boeing's most important plane is expected to finally fly passengers in 2027.

Over a dozen airlines are waiting on the yet-to-be-certified Boeing 777X, the world's largest twin-engine widebody in production, packed with more than 400 seats and capable of flying up to 9,000 miles nonstop.

Boeing says the wider cabin, longer wings with folding wingtips, and giant General Electric engines create more seat room, open new routes, and provide a more efficient and comfortable flying environment compared with the planes it's replacing.

After years of delays, Boeing executives reiterated in a July earnings call that the larger 777-9 variant is still on track to enter service next year. German flag carrier Lufthansa is the launch customer.

The giant new jet has become increasingly important. Airlines have baked it into plans to capitalize on booming demand for premium travel, expand their long-haul networks, and replace aging widebodies that are more expensive to operate. And Boeing needs a win — and the money — after years of diminishing trust among customers and the public.

But the 777X is far behind schedule. Boeing initially planned to deliver the jet in 2020, but a string of quality-control and safety problems at the planemaker have increased regulator scrutiny and slowed certification. Some buyers — notably Emirates — have voiced frustration over the cost of keeping older jets flying while they wait, and have declined to take delivery of some of the earliest-built 777Xs.

Still, airlines have placed more than 560 orders collectively, worth billions of dollars, for the passenger 777X variants.

At the UK's Farnborough International Airshow in July, Business Insider experienced a mock-up of the 777X cabin and spoke with Boeing to learn more about what flyers can expect onboard, assuming the jet launches as intended next year.

"Boeing wanted to produce a cabin that would be distinctive, that would be unique, that would be something that an airline could say 'we have something different to offer,'" Boeing's managing director of product marketing, analysis, and competitive intelligence, Justin Hale, told the media. "We believe we've done that with the 777X."

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From new lighting features and strategic door placement to optimize premium cabins, here are the coolest things we saw.

While the double-decker Airbus A380 and Boeing 747 remain the largest and widest passenger planes in service, the 777X boasts more cabin space than its 777 predecessor and the competing Airbus A350.