Plant 42 is a United States Air Force facility. It is the Antelope Valley's second-largest employer, and is considered a Geographically Separated Component of Edwards Air Force Base, 23 miles northwest of the airport. Most of the facilities are owned by private contractors and serve as a manufacturing plant for aircraft used by the United States and their allies' militaries. Plant 42 has 3.2 million square feet of industrial space and has a replacement value of $1.1 billion. Some of the plant's work involves production of spare parts for military aircraft, with other projects including maintenance and modification of aircraft such as the B-2 Spirit bomber and production of the Global Hawk and other unmanned craft. Aerospace contractors at Air Force Plant 42 share a common runway complex and either lease building space from the Air Force (commonly referred to as GOCO, or Government Owned Contractor Operated), or own their own building outright. There are eight separate production sites specially suited for advanced technology and/or "black" program projects. Currently the most well–known contractors at Plant 42 are Boeing, Lockheed Martin (home of the legendary Skunk Works), and Northrop Grumman.