Noi Bai grew out of a military airfield built north of Hanoi in the 1960s and used by the North Vietnamese air force during the war. It opened to civilian traffic in 1978, three years after reunification, replacing the older Gia Lam airfield closer to the city center. For its first decade it was bare bones, serving a country largely cut off from Western air travel.
The Doi Moi economic reforms from 1986 opened Vietnam to foreign airlines and outside investment, and Noi Bai grew fast from that point on. A new international terminal opened in 2001. The real change came with Terminal 2, paid for with Japanese development aid and opened at the end of 2014. A second runway, a new expressway, and a bridge across the Red River followed soon after.
Vietnam Airlines uses Noi Bai as its main northern hub, meaning an airport it uses to connect passengers between flights. VietJet Air and Bamboo Airways add heavy domestic capacity of their own. The Hanoi to Ho Chi Minh City route is one of the busiest domestic air corridors anywhere in the world. More new terminals are already planned to push capacity well past today's current limits.