On Friday, Boeing (BA 2.16%) announced  the completion of a contract whereby it will make upgrades on 68 F-15S fighter jets for the Kingdom of Saudi Arabia. The contract, valued at $4 billion, appears to be part of a deal announced in January, whereby Boeing agreed to sell the kingdom 84 new F-15SA fighters, and upgrade a further 70 existing F-15Ss to the F-15SA configuration. The "S" in both names refers to a specific model of the venerable F-15, preferred by the Saudis.

This contract, in turn, is part of a deal signed in 2010, whereby the U.S. agreed to sell some $90 billion worth of military hardware -- America's biggest defense contracting sale to a foreign country in modern times. That deal included the sale of products manufactured by everyone from Boeing to Lockheed Martin (LMT -0.69%) to General Dynamics (GD -0.20%) and General Electric (GE -1.31%) -- to the Saudis.

Work on the F-15 upgrades will be performed at Robins Air Force Base in Georgia and is expected to conclude by the end of 2019. In addition to selling and upgrading planes, Boeing will supply four "base stand-up kits," including spare parts and support equipment necessary to operate the new planes, which will fly out of four bases in Saudi Arabia.