Devil's Advocate
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Seems like it was a US only fleet history that was broken at a time when United was Boeing’s biggest customer.United dropped their “all-Boeing” fleet strategy in 1931 with the introduction of the Ford Tri-Motor. At no point in the jet era has United ever had an all-Boeing fleet.
https://www.latimes.com/archives/la-xpm-1992-07-09-fi-2575-story.html