FARNBOROUGH, England — The US Defense Department is hammering out the final details of its Block 4 upgrade plan for the F-35 joint strike fighter, but the Marine Corps has made clear that Raytheon's Small Diameter Bomb II (SDB II) is at the top of its wish list.

Asked by journalists at the Royal International Air Tattoo what he'd most like to see in the modernization program, Marine Corps Lt. Gen. Jon Davis, deputy commandant for aviation, characterized the SDB II as the most critical system.

"That's a fantastic weapon," he said July 8. "I want to get it on there and really increase the capability of the airplane."

Days later at Farnborough International Airshow, Davis' executive assistant, Col. William Lieblein, reiterated the service's desire for the SDB II. The service also wants to incorporate full-motion video onboard the F-35 and improve the electro-optical targeting system with forward-looking infrared, he said.

The F-35 will be initially equipped with the first iteration of the weapon, but the follow-on version includes a tri-mode seeker that uses infrared, millimeter wave and laser guidance to identify and destroy targets.

Raytheon and the Air Force recently started up a new round of SDB II flight tests of the weapon's coordinate attack and laser modes, the company announced Monday. When coordinate attack mode is engaged, the SDB II's GPS system will direct it to fixed targets at distances of more than 40 miles, while the latter mode uses a semi-active laser to illuminate targets.

For the most part, the services have finalized which capabilities will funnel into the Block 4 modernization program but are deliberating when those upgrades will funnel into production, Davis said.

A capabilities development document is working its way through the Air Force and will go to the Joint Requirements Oversight Council later this summer, said F-35 program executive officer Lt. Gen. Christopher Bogdan.

"We have a pretty good idea of what's going to be in the first few increments of Block 4," he said in a July 9 interview. "Now we're working on putting together a funding profile to meet that and the acquisition strategy to do that."

The current plan is to separate the Block 4 upgrades into four increments. Those capabilities will be integrated into the F-35 every two years, starting in 2018 with the first delivery of new capabilities in 2020.

"If a capability is not mature as we develop it, instead of waiting around for it, we'll push it to the next increment," he said. "So we're trying to work the strategy with industry on how to be that flexible with contracting on the business side."

The F-35 joint program office (JPO) estimates the upgrades will cost somewhere around $3.8 billion to $4.8 billion to procure and integrate into the aircraft. That doesn't include unique international requirements such as Norway's joint strike missile. All weapons, including SDB II, will be fielded within the first two increments, Bogdan said.

Other upgrades will include more modern electronic warfare systems, radar, avionics and interfaces, and changes that improve the aircraft's reliability, maintainability and ability to deploy.  Generally speaking, the JPO will look to current contractors for Block 4 systems, but could compete capabilities if technology has significantly advanced.

"One of the big things to drive cost down in Block 4 is that we will be looking to outside companies who may not have traditionally had equipment on the airplane," he said. "That's because in Block 4 we're getting computers that are open and modular, an open system, so that you can put new sensors and new things on the airplane easier than having to change the whole infrastructure."

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