The British-built Kestrel was the prototype version of the Harrier and became the world's first operational VSTOL fighter with the RAF in 1969. As XS688 this was one of six Kestrels that came to Edwards AFB for US armed forces testing and subsequent entry into service with the US Marine Corps. This example is now on display in the R&D Gallery of the NMUSAF.

Initially flew as XS688 with the Tripartite Evaluation Squadron before being allocated to the USA for further tests.

One of 9 Kestrel FGA.1s built as development aircraft between the experimental P.1127s and the production Harriers, first flying in 11.64 as XS694, then being transferred to the US Army for evaluation as 64-18266. Apparently became the second NASA 520 when the first (ex XS694/64-18268) crashed, substantial parts from the latter being used to make it airworthy again for NASA research use at Langley. They also had NASA 521 (ex XS689/64-18263, now displayed at the nearby Virginia Air & Space Center