US and Israeli diplomats announced the largest military assistance package in US history on 14 September in a $38 billion, 10-year package that will deliver an assortment of American-designed weapon systems, including Lockheed Martin F-35 and Boeing F-15 fighters to Tel Aviv.

The agreement, which takes effect on 1 October 2018, pledges $38 billion to Israel over the next decade, including $33 billion in foreign military financing funds and $5 billion for missile defense, US national security advisor Susan Rice said.

In a surprise move, Rice added to the F-15 to the mix of weapon systems on the agenda for the 10-year aid package, perhaps reducing funds available to purchase Israel’s planned complement of 75 F-35I Adirs.

“This additional funding will allow Israel to update the lion’s share of its fighter aircraft fleet, including the acquisition of additional F-35s and F-15s,” she says.

Boeing referred questions about the agreement to the US and Israeli air forces, who were not immediately available for comment.

As Israel prepares for the delivery of the first F-35I Adir to Nevatim Air Base in December, the inclusion of F-15s in the deal could affect the mix of F-35s and F-15s in their fleet. While Israel defence officials requested more F-15 squadrons in 2015 as part of a “compensation package” following the US’s decision to lift sanctions against Iran, industry sources later told FlightGlobal that acquiring additional F-15s was unlikely.

Still, the agreement provides solid backing for the F-35 programme from Israel. The US has approved the sale of up to 75 F-35s to Israel and the country has signed contracts for 33 F-35s, the last of which will arrive around 2021, FlightGlobal previously reported.

During the unveiling of the F-35I Adir in Texas earlier this summer, Israeli air force chief of staff Brigadier Gen. Tal Kelman recommended the purchase of 17 additional F-35As. In addition to the conventional takeoff variant, Israel is also mulling over the short takeoff and vertical landing (STOVL) F-35B, but that purchase would depend on additional F-35A buys. While Israeli officials would not comment this week on the MOU’s effect on discussions for a STOVL variant or a renewed V-22 purchase, Israeli Prime Minister Benjamin Netanyahu hinted at the need for a vertical takeoff aircraft in July.

The memorandum of understanding, signed by Israeli national security adviser Jacob Nagel and US under secretary of state for political affairs Tom Shannon, comes after intense negotiations between the two nations. As the US faces tightening budgets and a gridlocked Congress, the agreement promises an unprecedented aid package and a steady stream of funding over the next 10 years that will be protected from political turmoil.

“For years, US funding for missile defense has been subject to uncertainty of the annual appropriations process,” Rice says. “Some years the funding has been unclear for months at a time, some years it’s been in decline. With this funding [Israel] will be able to count on a steady multi-year commitment.”