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In page Boeing KC-135 Stratotanker:

"
  • 63-7998 – KC-135A on static display at the Pima Air & Space Museum, adjacent to Davis-Monthan AFB, Arizona. It was operated by NASA as N931NA and is the second of their two research aircraft.[21]
  • 63-8005 – KC-135A on static display at Grand Forks AFB, North Dakota.[citation needed]