Senior Pentagon officials voiced frustration about the pace of Lockheed Martin Corp’s development of the $396 billion F-35 Joint Strike Fighter program at a high-level review on Friday, according to several sources familiar with the program.

Officials did not approve a comprehensive plan for operational testing of the F-35 program as had been expected.

The Pentagon’s Defense Acquisition Board huddled for more than four hours on Friday evening in a meeting described by one participant as “very painful” given ongoing challenges facing the high-tech F-35 helmet that is integral to the craft’s weapons systems, and other aspects of the huge program.

Marine Corps Commandant General James Amos told Reuters in an interview on Saturday that he had not yet been briefed on Friday’s meeting, but was closely following work on the helmet, since its completion was needed soon to allow the Marines to become the first service to use the new jets operationally.

“The helmet is a critical piece that needs to be solved,” Amos said, noting that the Marines urgently needed the short takeoff, vertical landing (STOVL) version of the plane to replace their aging fighter jets, which include older model F/A-18 Hornets built by Boeing Co..

The Marine Corps initially hoped to start using the new F-35B jets this year, but a series of program restructurings has pushed that date back several years.

If the helmet being developed by Vision Systems International (VSI), a joint venture between Israel’s Elbit Imaging and Rockwell Collins succeeds, it will be the most advanced ever built.

It is supposed to let pilots see data from all the plane’s sensors, effectively allowing the pilot to look right through the floor of the plane and all around it. But the project has run into problems with night vision, delays in displaying data, jitter under certain conditions, and more recently, a green glow at the visor’s edges and problems with alignment.

Lockheed Martin has brought in an alternate contractor, BAE Systems, to work on a substitute helmet in case the VSI helmet does not meet its deadlines. Current F-35 program manager Navy Admiral David Venlet is meeting with BAE officials during a trip to Britain and Italy this week.