The deal will be made up of 240 planes sold directly to airlines and 60 aircraft destined for leasing companies, the company said in a news release.
China Aviation Supplies Holding, ICBC Financial Leasing and China Development Bank Leasing inked the agreement after Xi's arrival in Seattle, the Chinese news agency Xinhua reported.
Xi told hundreds of Boeing employees at the factory that he sees bright prospects for future collaboration between China and Boeing.
Boeing and a state-owned Chinese partner, Commercial Aircraft Corporation of China, will also build an aircraft assembly plant in China, where the 737 jet will be produced. Known as a completion center, it would finish, paint and deliver 737s built in the US.
Wednesday's deal was quickly lambasted by Republican presidential front-runner Donald Trump, who said the planned new facility in China could hurt American jobs. But the company said its plans in Asia would not lead to layoffs at home.
Analysts say Boeing is attempting to match its European rival Airbus' presence in China, after it opened its first assembly line outside of Europe in Tianjin in 2008. In July, Airbus agreed to build a second completion and delivery centre for A330 jets in China.
Boeing predicts within 20 years, the Chinese market will be worth around $950 billion.
Shares in the jet manufacturer fell 1.5 percent to $132.05 in afternoon trading in New York on Wednesday.