The F-35 Joint Strike Fighter reached the highest monthly total of test points yet during June, Lockheed Martin [LMT] said yesterday, in a positive step for a program that has been plagued by cost overruns and delays.

Lockheed Martin said that 1,118 test points were carried out last month. That included the first flight of the Marine Corps’ F-35B short-takeoff and vertical-landing (STOVL) version with the latest software.

The Navy’s F-35C carrier variants’ first night flight was completed at Naval Air Station Patuxent River, Md., and the Air Force’s F-35A carried out weapons pit drop testing at Edwards AFB, Calif., Lockheed Martin said.

Lockheed Martin said June marked the 18th straight month that the F-35 test program remained ahead of schedule, and that so far this year, as of June 30, the Joint Strike Fighter has conducted 595 test flights, 150 more than planned.

The largest acquisition program in the Pentagon’s history, the F-35 program has been subject to widespread criticism for massive cost overruns and delays in production and deliveries. The Pentagon plans to procure 2,443 F-35s for the Air Force, Navy and Marine Corps at an estimated cost of $395 billion–about $117 billion from the expected cost five years ago.

Earlier this year, the Defense Department restructured the program for the third time, deferring the purchase of 179 planes over the next five years for an anticipated $15 billion in savings.

The Government Accountability Office (GAO) said in a June report that it expected redesign and modification–known as concurrency–costs in the F-35’s development to remain “elevated” into 2019, the first year the fighter jet is scheduled to go into full production.

Meanwhile, Lockheed Martin also said yesterday four F-35s have been delivered to the Pentagon since June 29, marking the first time the Defense Department has more operational F-35s than test aircraft. A total of nine have been delivered so far this year, giving the department 30 of the airplanes. Sixteen of them are operational aircraft, Lockheed Martin said.