Flight, 16/12/60: "first details of the Boeing 727. This aircraft is in all major respects a carbon copy of the D.H.121 Trident. Last February (having bought DH, 17/12/59, HSAL's) Sir Roy Dobson and Sir Aubrey Burke visited Boeing to discuss, among other things, collaboration on the D.H.121. Boeing were interested, and four senior DH engineers, including the chief designer Mr C. T.Wilkins and chief technician Mr. D. R. Newman, went to Seattle also. Boeing engineers later visited Hatfield, and other exchanges followed.
(Roger Bacon) would not blame DH for feeling...disenchanted with the "collaboration" that has resulted. I do not know whether they feel sore or not: I think they have been in the game long enough to have few illusions left to be shattered. But I think I know one thing : henceforth, whenever the Minister exhorts DH—or anyone else in our industry—to collaborate with a foreign industry, he will be reminded of what happened when we tried it with the Trident."
That is the origin of the perception that Boeing lifted the Trident. As philbky says, Medway/727 was in hand by mid-59, as a logical adaptation of 707/720. For the owners of Renton and Wichita, a Hatfield shop visit would add nothing to corporate knowledge; the ex-DH Lofting Office would at that time have little to delight them. HSAL's intent was collaboration, to win US customers - lessons from Comet 3/PAA were that the pond was a very effective moat. One writer (can't now find it) has implied that the Knights' deal-making foundered.