They were built from B-57s built by Martin which was similar to the RAF design. The longwing versions of which these are, were rebuilds or re-manufactured from the Martin units by General Dynamics. I could look up the numbers and see which ones NASA has now. One of the ones they have is originally tail #503 which we had at Rhein-Main AFB in Germany in 1967.
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You beat me to it but I'll expand a bit. The English Electric Canberra was an all wood airframe. Martin built the B-57s. I crewed EB-57E's, these were equipped with tons of electronic countermeasure equipment and zero armament. The WB-57s were originally modifications for weather service, not sure if NASA uses them for that but probably. Little known was the RB-57 which had a huge stretched wing and was for high altitude reconnaissance.
The B-57 was really fighter like akin to a B-25. They were actually very aerobatic and maneuverable in their basic dress. Our squadron commander Col John Harris of the 4713DSES did some interesting maneuvers with a stripped out EB when we were deactivating that squadron. It was very impressive to see and I especially remember the snap roll over the runway, it was spectacular!!
In case you can't tell, I loved this aircraft, I also loved the F-106s I crewed after this and equally hated the B-52s I was forced onto later.
Here's and RB and an EB (A model)
Added a neat 3 view showing the standard wing and the RB wing