The Pentagon’s chief weapons tester on Friday sent an 18-page report to the US Congress detailing several problems with the F-35 fighter jet being developed by Lockheed Martin.

In October 2010, Israel purchased 19 of the radar-evading fighters for $2.75 billion, with an option to increase that total to 75. Israel’s order was initially slated for a 2015 delivery, but that date is appearing increasingly unlikely.

According to Reuters, the report listed software development, weapons integration and the helmet needed to fly the plane among the issues that highlight the “lack of maturity” of the Pentagon’s most expensive weapons system ever, which has seen many budget overruns and is estimated to cost US $396 billion.

While the fighter jet has completed over one-third of its flight tests, many of the problems, some previously known but unresolved and others newly discovered, have prevented Lockheed Martin from conducting some of the further testing scheduled.

Lockheed is building three models of the F-35. Since the inception of the project in 2001, the jet has been restructured three times in order to slow down production and to allow more progress on the development program.

Reuters reported that neither the Pentagon’s F-35 program office nor Lockheed Martin had any immediate comment about the report.

In July 2012, the Pentagon and Lockheed Martin Corp. reached a $450 million agreement with Israeli companies, including Elbit Systems Ltd. and Israel Aerospace Industries, to integrate Israeli systems into the F-35.