The AWACS, NATO's Reconnaissance Air Wing

NATO is not an army in its own right, its member states provide the national forces that defend Europe under its banner.

But the Atlantic alliance has its own air wing, 14 Boeing E-3A Airborne Warning & Control System (AWACS) aircraft, which it was keen to display in the week running up to the NATO London summit.

The distinctive plane -- a modified 707 airliner with a huge radar dome like a flying saucer -- is getting a bit long in the tooth and due for a million-dollar refit.

And what better way to reassure US President Donald Trump that Washington's allies are ready to up their defence spending than by letting Boeing announce the contract just before the summit.

The NATO Airborne Early Warning and Control Force is based in Geilenkirchen, Germany.

The planes began NATO service in 1982 at the height of the Cold War and are due to be phased out by 2035, having served to help the alliance monitor the wars that followed the break-up of Yugoslavia.

While the force's core mission, like NATO's, is the defence of Europe it has been used in support of Western forces in the first Gulf War and now patrols the skies over Turkey to keep an eye out for spillover from the Syrian conflict.

It was sent to the United States in the aftermath of the 9/11 attacks and also provides aerial security of NATO summits, like the one that will take place next week in Watford, outside London.