I'm not with Laminar Research, just a fellow simmer in the UK, but as there does not currently appear to be an X-Plane 11 version available for mobile, I presume that you are referring to X-Plane 10 mobile?
If so, the SR-71 does not seem to be available for X-Plane 10 mobile, as you will see on the following URL:- https://www.x-plane.com/aircraft/platform/mobile/. It is available for X-Plane 11 desktop however!
According to J Roberts of LR, "X-Plane 10 Mobile is fairly in sync with X-Plane 11 for desktop PCs. They share code back and forth already, so even though the name says "10" it's up to date with 11."
Having said that though, I guess that there must be some restrictions for the mobile version, given that the complete desktop version is very system hungry.