An old F/A-18 Super Hornet fighter jet is being refurbished at a hangar at Boeing's St. Louis headquarters, the first of many planes that can now be refitted as the Pentagon spends the biggest budget in its history.

Boeing executives and members of the US Navy will celebrate with an induction ceremony on Thursday.  

The Super Hornet, at the end of its original lifespan, was flown by the US Navy Gladiators Strike Fighter Squadron based out of Virginia Beach since 2004.

Boeing will perform updates and maintenance on the plane and three others at a cost of 73 million US dollars to keep them in the fleet for another dozen years.

Boeing is also making new Super Hornets under the Pentagon spending plan.

The company has a contract to deliver 134 F/A-18 Super Hornets to the US Navy over the next few years.

Each new airplane costs more than 70 million US dollars.

"This contract is very important to Boeing's operations in St. Louis," said defence industry consultant Loren Thompson of The Lexington Institute.

"Historically St. Louis was the most important production centre for fighters in the United States. However it has been declining and with this contract it will stabilise at least through 2025."

Boeing also sells numerous fighter jets to other countries, such as Saudi Arabia.

The US Marines and Air Force prefer Lockheed Martin's newer F-35 stealth fighter plane, which uses technology to hide from radar.

The F/A-18 has been in use by the US Navy on air craft carriers for decades.

"Demand for more military spending has been building up since 2011," said Thompson.

"That's when Congress passed the Budget Control Act which capped defence spending. As a result there are readiness and modernisation requirements that haven't been met and the Trump administration committed itself to increasing military spending as soon as it came to office."

At 700 billion US dollars, the US defence budget is larger than any other country.

The biggest winners are the country's largest defence contractors, Boeing, Lockheed Martin, and General Dynamics.