Fighter jet maker Lockheed Martin said it will buy helicopter manufacturer Sikorsky for $9bn (£5.78bn).
It will be the biggest purchase for Lockheed, which oversees the F-35 fighter project, since it bought Martin Marietta in 1995.
Sikorsky makes the Black Hawk helicopter and the Sea King, which was made by UK firm Westland under license.
United Technologies, which owns Sikorsky, said last month it planned to sell the firm.
United Technologies will return the spoils to shareholders by buying back shares, it said.
The deal will only demand a net cost to Lockheed of $7.1bn, because of $1.9bn of tax benefits it will make, it said.
The deal will be closely watched by US defence authorities, who will want to ensure competition is maintained in the market.
Lockheed also said it will review its government IT and technical services businesses.