(Editors Note: This story has been updated to clarify the scope of the F-35B internal weapons bay design changes.)

The internal weapons bay of the F-35B Joint Strike Fighter cannot fit the required Small Diameter Bomb II weapons load, and a hydraulic line and structural bracket must be redesigned and modified ahead of the planned Block 4 release in fiscal year 2022, the joint program office confirmed this week.

The Air Force and Raytheon plan to begin scaling up production of the 250-pound class, precision-attack munition, except the current F-35B internal weapons bay cannot fit four of the eight required SDB IIs in its current configuration.

The Marine Corps is purchasing 353 of the F-35B jump jets and 34 had been delivered as of Feb. 2, according to a fact sheet from prime contractor Lockheed Martin. JSF partners Italy and the United Kingdom are also procuring F-35Bs and three of those international orders have been satisfied.

In response to questions from Inside the Air Force, F-35 spokesman Joe DellaVedova confirmed the weapons bay does not currently meet the requirements to house the planned Small Diameter Bomb II load and is being redesigned and modified in line with the scheduled rollout of Block 4 capabilities.

The short-takeoff-vertical-landing (STOVL) aircraft has unique weight requirements compared with the Air Force's conventional F-35A and Navy's F-35C carrier variant because of its vertical lift fan and it has a smaller internal weapons bay.

According to DellaVedova, the JSF program has been aware of the issue for some time and expects to award Lockheed a contract later this year to complete the design changes. The F-35 is designed to carry eight precision-attack small diameter bombs internally and 16 externally on its wings, and the program office has not publicly acknowledged the issue.

The issue surfaced this month in budget documents accompanying the Air Force's FY-16 budget request. A line in the SDB II program scheduled is titled "SDB II Redesign for F-35B/C," but DellaVedova confirmed the weapons bay issue only relates to the STOVL version.

The JPO is targeting to have the F-35B weapons bay changes incorporated into the post-systems development and demonstration airplanes delivered in the 2019, 2020 time frame and beyond, DellaVedova said in a Feb. 25 phone interview with ITAF. "There are considerations for small bay changes to support the rest of the Block 4 weapons suite. Rather than make multiple small changes, we're planning to do one modification that will address all Block 4 requirements," he said.

The Air Force's Small Diameter Bomb II program is worth around $4.1 billion and will deliver around 17,000 weapons, including 5,000 for the Navy. The sophisticated, 250-pound glide weapon is produced by Raytheon and will eventually replace the legacy version built by Boeing.

SDB II integration with the F-35B will not impact the Marine Corps' preparation for initial operational capability in July, but would become a problem as new weapon systems are introduced. The F-35 is designed for stealth and carries weapons internally to reduce its radar cross section.

The Navy initially wanted to field the SDB II first on the F-35B/C but is instead bringing forward integration with the F/A-18 Super Hornet. The SDB II is an F-35 Block 4 software capability and the release of that software load has been pushed back to FY-22.

In a Feb. 24 phone interview with ITAF, Raytheon's SDB II business development executive Jeff White said F-35 fit checks have found the weapon system fits fine in the larger F-35A/C weapons bays.

"I think it's just the B," White said. "When we did our fit checks on the JSF, the A and the C basically had the same bay."

According to White, the SDB II program is nearing the end of its development phase and a production decision is due out soon. He said the SDB II form will not be altered to suit the F-35B and it is hoped the weapons bay redesign will wrap up in 2016.

White said SDB II is an important aspect of the JSF program, and the international partners are eager to receive the new weapon along with the Air Force and Navy. He expects production will ramp up to more than 4,000 munitions per year in the early 2020s.

The United Kingdom is the second largest F-35B customer behind the Marine Corps, with 138 aircraft. Italy wants 30 of the jump jets to augment its F-35A fleet.

According to DellaVedova, the United Kingdom has not yet committed to purchasing the SDB II but is aware of the weapons bay issue. He said Italy would have also been informed.

"This is not a new issue to us," DellaVedova said. "We've been working with the SDB II program office and their contractors since 2007. The fit issues have been known and documented and there were larger and more substantial modifications needed to support SDB II that have already been incorporated into production F-35 aircraft. These minor or remaining changes were put on hold until the aircraft reached a sufficient level of maturity to ensure that the needed changes would not adversely impact any ongoing SDB [II] developments."

In a Feb. 2 press briefing on the FY-16 president's budget, Deputy Assistant Navy Secretary for Budget Rear Adm. William Lescher said the service planned to reduce its procurement of SDB II by 750 across the future years defense plan to 1590 units.

Lescher said this change in the procurement profile aligns with the planned F/A-18 Super Hornet integration in FY-20 and new F-35 Block 4 software rollout date of FY-22.

ITAF reported in December that the Office of the Secretary of Defense directed the Navy to bring forward the planned integration of the SDB II on the F/A-18E/F. The Air Force's threshold aircraft for SDB II is the F-15E Strike Eagle.

Other weapons tied to the F-35 Block 4 software release include the Joint Stand-Off Weapon, Norwegian Joint Strike Missile, Turkish Stand-Off Missile and AIM-9X Sidewinder Block II. -- James Drew