Boeing Co. is inspecting more than 400 stored 737 Max jets after discovering debris such as tools or rags left in the fuel tanks of several newly built but undelivered aircraft.

Boeing Co. failed to sell any commercial planes in January, extending a slump that has strained the company’s finances since two deadly crashes grounded the best-selling 737 Max last year.

Interest in Boeing’s 747-400 freighter family already was rebounding last year, even as Delta Air Lines Inc. and United Continental Holdings Inc. hosted nostalgic farewell tours to mark the end of U.S. passenger service on the four-engine behemoth nicknamed the “Queen of the Skies.” With Boeing’s factory-fresh models sold out through 2021, cargo carriers are snapping up jumbo freighters that were built from 1993 to 2009 — if they can find them.

Boeing Co. has a temporary plan to save the iconic but slow-selling 747 jumbo jetliner: buying its own planes and leasing them to cargo haulers.

FedEx Corp. agreed to buy 50 Boeing Co. 767 freighters with a list value of about $10 billion, the model’s largest-ever order, as it upgrades the fleet at the FedEx Express unit.