Boeing [BA] has received a $139.8 million contract to build four MH-47G Block II Chinook helicopters for U.S. Army Special Operations Aviation Command.

The heavy-lift helicopters mark the start of Block II production for SOCOM that is expected to run into the late 2020s.

The Army has 69 MH-47G Chinooks in operation and eventually will upgrade all of the highly classified aircraft to Block II configuration, an evolutionary upgrade package Boeing developed and is now producing under a $276 million contract awarded by the Army in 2017.

The first of three U.S. Army CH-47F Chinook Block II test aircraft was loaded into final assembly in late June, marking a major milestone in the effort to dramatically overhaul the Boeing-built heavy-lift helicopters. MH-47Gs are built to SOCOM standards and are equipped with classified avionics, sensors and weapons. Both models are built by Boeing at its manufacturing facility outside Philadelphia.

“The Army uses the MH-47G for some of its most difficult and challenging missions,” said Pat Donnelly, Boeing’s director of H-47 domestic and foreign military sale programs. “By incorporating key Block II features we significantly extend the service life, enhance performance, and maintain Chinook’s position as the world’s preeminent special operations helicopter.”

Block II was designed to achieve specific performance enhancement metrics, including a 22,000-pound payload and high/hot hover performance at 4,000-feet on a 95-degree day. Maximum takeoff weight is boosted to 54,000-pounds with the goal of carrying a Joint Light Tactical Vehicle.

First delivery of a production Block II Chinook to the Army is expected in 2023. The Army plans to eventually upgrade more than 500 Chinooks — its entire fleet of CH-47Fs — to Block II configuration. The Army is still in the process of bringing all of its Chinooks to F-model configuration. Boeing will bring in an estimated $14 billion-plus to change the Army’s entire fleet to F-model configuration.

Included in the upgrade package are improved avionics, speed enhancements and a beefier drivetrain that will transfer greater power from the 20 percent more powerful Honeywell [HON] T55 engines to all new, swept-tip advanced Chinook rotor blades. Without any other upgrades, the blades are designed to provide an additional 1,500-pounds of lift.