The most inland flight is Chicago’s O’Hare International Airport to Addis Ababa Bole International Airport, a route flown three times a week by Ethiopian Air; no nonstop flights depart from the West coast. The longest-distance flight currently is Delta’s Atlanta to Johannesburg route (8,440 miles and 15 hours).

The reason for the boost in nonstop traffic between the continents has as much to do with technology as it has to do with demand.

Longer flights — really long flights — are increasingly flown by airlines all around the world. Developments in plane construction makes it possible to fly longer routes, and record-breaking “ultra long-haul” flights now take passengers between New York City and Singapore (Singapore Airlines; 9,500 miles); Doha and Auckland, New Zealand (Qatar Airways, 9,000 miles); and London and Perth, Australia (Qantas; 9,000 miles). In August, Qantas announced it was testing a 19-hour New York City to Sydney flight, which would be the world’s longest nonstop commercial flight.

The Boeing 787-9 aircraft in particular is being eyed as the key to opening up these longer nonstop routes. That plane can “make flying long distances more economical for airlines, and requires fewer passengers flying a route to do so,” said Gary Leff, a travel industry blogger with the site View from the Wing.

At the same time, for airlines based in the United States, the marginal gains in international markets may be greater than in the domestic market. “The domestic market is mature, and profitable opportunities for growth are limited, though not nonexistent,” Mr. Leff said. He believes demand for flights like United’s new Cape Town route will come primarily from passengers based in the United States.