Boeing predicts surging demand for air freighters as cargo shipments double in the next 20 years amid a growing reliance on e-commerce that's reshaping the American economy.

Boeing expects freight traffic to climb by nearly 5 percent annually over the next two decades, pushing air cargo operators like Memphis, Tenn.-based FedEx and Bonn, Germany-based DHL to purchase more than 2,600 freighters. Some 980 new medium and large freighters and 1,670 converted freighters will go toward replacing older airplanes and expanding the global fleet, according to the Chicago-based planemaker's World Air Cargo Forecast.

"The air cargo market continues to be a major element of commercial aviation's growth story," Darren Hulst, managing director of market analysis and sales at Boeing, said in a statement. "Our new forecast indicates strong long-term air cargo trends, which coincide with the market recovery that we have seen over the last few years across Europe, North America, and Asia."

[Also read: Boeing reaches $13 billion in defense wins with T-X training jet deal]

The company expects e-commerce shipments to increase 20 percent a year to roughly $5 trillion by 2021. In order to meet the demand, the Chicago-based manufacturer estimated that freighter fleets will grow by nearly 75 percent, from 1,870 airplanes to 3,260. Boeing said it sold 128 freighters in last 22 months.

"We have invested in our freighter family to help express cargo and general freight operators carry out their missions around the world," Hulst said.

Air cargo has become an increasingly important source of income for airlines, too: Freight revenue rose 18 percent in the first half of 2018, according to the lobbying group Airlines for America.

Delta Air Lines said earlier this month that revenue from air cargo grew 18.5 percent in the three months through September. At United Airlines, cargo revenue grew 6.1 percent in the same time period to $296 million. Overall profits climbed to 30 percent to $836 million, the Chicago-based carrier said on Wednesday.