HANOI—Vietnamese air carriers signed more than $21 billion in airline orders and service contracts with U.S. companies Wednesday on the sidelines of this week’s summit between President Trump and North Korean leader Kim Jong Un.

Some of the deals had been announced before. VietJet Air’s plans to buy 100 Boeing 737 Max jets valued at $12.7 billion was first unveiled at an air show last year. It added to the package a $5.3 billion agreement with General Electric Co. to provide and service the company’s Leap-1B engines. Bamboo Airways, a new player in the Vietnamese market, will buy 10 Boeing 787-9 Dreamliner jets valued at $3 billion.

Representatives of the Vietnamese carriers and U.S. firms signed the deals with Mr. Trump and Vietnam’s President Nguyen Phu Trong looking on, underscoring the growing commercial relationship between the former wartime foes. The U.S. is now Vietnam’s single biggest export market, and the communist state’s economic boom in recent years has prompted some diplomats to hold it up as a model that North Korea might one day follow.

The deals would support American jobs and provide increased safety and reliability for Vietnamese international travelers, a senior administration official said.

A surge in air travel, led by private companies such as VietJet and Bamboo Airways, is one of the most visible signs of how Vietnam has changed. Airline executives say they are optimistic they will soon be approved to fly to the U.S. too.