A few days ago I posted about Pratt&Whitney's newest flying testbed, a modified Boeing 747SP used for testing new aircraft engines. https://plus.google.com/114133424228405038490/posts/iGWatNAKwcU
As some commenters mentioned, this plane was by no means the first of its type. At about the same time that Pratt was initiating its new testbed, it retired an old one. The plane being put out to pasture is actually the last remaining flying Boeing 720 in the world. The 720 was a shorter version of the ground-breaking 707 and now they are all gone.
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Farewell to Boeing's 720
Aviation Week
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