Lockheed Martin has finally begun delivery of the latest lot of F-35 Joint Strike Fighter aircraft to the Pentagon after a roughly six-month delay.

The Pentagon has accepted delivery of three conventional-takeoff-and-landing (CTOL) variants and one short-takeoff-and-vertical-landing (Stovl) version, says Marillyn Hewson, the incoming chief operating officer of Lockheed Martin, F-35 prime contractor.

The low-rate-initial-production (LRIP) 3 aircraft were slated for delivery by the end of 2012. All 17 of the stealthy fighters in this lot— two F-35Bs for the U.K., one F-35A for the Netherlands, seven F-35As for the U.S. Air Force and seven F-35Bs for the U.S. Marine Corps — are off of the company’s Fort Worth assembly line.

With the Dutch government in “caretaker status” in advance of national elections, the socialist party has pushed the country to back off of its commitment to the F-35. However, a conservative win would likely reinstate support for the program. Thus, the commitment from the Netherlands remains shaky.

The LRIP 3 aircraft had been awaiting official acceptance via the official “DD250” process managed by the Pentagon, says Vice Adm. David Venlet, the F-35 program executive officer. He said last month that the paperwork was taking longer than planned for the turnover process, which he says is “not unnatural” early in the production process.

The four new aircraft, which include the Block 1B software, will join 12 already at Eglin AFB, Fla., Hewson says. The Block 1B software will feature a new multi-level security package.

Six of the F-35As at Eglin are being used for an operational utility assessment this summer; pilots will fly the aircraft to test out a rudimentary pilot-training syllabus. The results of this assessment will determine whether the Air Force is comfortable giving the nod to start official pilot training. That is not expected to happen before 2013.