In April, Boeing provided a $425 million advance to support Spirit's liquidity. Spirit, Wichita's largest employer with around 12,800 full-time workers, previously disclosed it has spent all of that money, as well as the entirety of a $350 million bridge loan that was set up earlier this year.

Boeing has agreed to give Spirit AeroSystems , the company that develops aircraft fuselages, a hefty financial boost. The US planemaker will provide up to $350 million in advance to support the production of products as the supplier grapples with a liquidity crisis.