Northrop said Wednesday it received the request as FTC reviews the transaction as part of the regulatory process.
The request for additional information came a week after Orbital ATK shareholders voted to approve the merger.
Northrop will pay $7.8 billion in cash and assume $1.4 billion in net debt from Orbital ATK under the deal originally announced in September.
Northrop said it expects to close the transaction in the first half of 2018.