NATO has signed a $1 billion (£774 million) contract with Boeing to modernise its AWACS (Airborne Warning and Control Surveillance) reconnaissance fleet.
The alliance's Secretary General Jens Stoltenberg said the E-3A aircraft are due to be replaced after 2035 - before then, the 14 aircraft will be upgraded as part of the new contract.
Mr Stoltenberg was joined by President of Boeing International, Sir Michael Arthur, for a ceremony at Melsbroek Airport, Brussels, to mark the signing.
Also known as the "eyes in the sky", the fleet has helped to protect NATO airspace close to Russia since it annexed Crimea in 2014.
Inside, the planes are kitted out with technology capable of pinpointing any ship or aircraft more than 250 miles away.
Watch: What it is like during a mission on board an AWACS aircraft.
Identified by their distinctive radar dishes, the modified Boeing 707s routinely launch missions from their base at Geilenkirchen in Germany.
They currently watch skies close to Russian airspace around Poland and the Baltic States, with their 360-degree radar picture aiding jet fighters or other military users.
Exactly what will replace them in 16 years remains unclear.