The six MQ-8C aircraft ordered have greater endurance, range and payload capacity than the MQ-B and bring the number of the new variant ordered by the Navy to 14.
The Navy, which plans to procure 30 aircraft, will begin deploying the Fire Scouts next year.
"This contract provides significant momentum for the work Northrop Grumman and its supply chain partners are doing to meet the Navy's requirements," said George Vardoulakis, vice president for tactical unmanned systems with Northrop Grumman's Aerospace Systems sector.
"Our entire team is focused on delivering this game-changing capability on time, on cost and with unquestionable quality.
"Along with our industry partners -- Bell, Rolls-Royce, Cubic and others -- we are making significant progress in reducing cost, enabling us to achieve our affordability targets and provide the Navy with the absolute best value."
The contract for the six aircraft carries a value of $71 million.