The Pentagon has withheld payments of $7 million a piece on the first four F-35s upgraded by Lockheed Martin Corp., with the fighter jets placed in storage until testing shows its software delivers on new capabilities.

Lockheed is on contract to deliver an additional nine TR-3 jets a month with improved software and hardware, or as many as 45 more, by Dec. 31.

The aircraft needs the delay-plagued software upgrade to fully function with new cockpit hardware before it can carry more precise weapons and gather more information on enemy aircraft and air defenses. The “TR-3” upgrade will increase processing power 37 times and memory 20 times over the F-35’s current capabilities.

Read more: Lockheed Risks $400 Million Payment Delay Over New F-35 Software

The Pentagon’s F-35 program office and Lockheed will “ensure that all these aircraft are safely and securely stored” until they are formally accepted, office spokesman Russell Goemaere said in a statement.

Lockheed said in a statement that “we continue to produce F-35s at the rate originally forecasted for this year and have completed 77 TR-3 test flights as of July 31.”