They can if they need to... Both the U2 and the SR71 had a crazy short development time - months, not years. The history of both those aircraft from a build/design perspective is an amazing read. They knew the U2 would be obsolete for the original mission and got the blackbird in the air. Got to wonder if it is just significantly harder to hide things now than it was in the late 50's. Between satellites, cell phones, and globs of potential collaboration - it would be hard to keep something in the air a secret.

http://www.roadrunnersinternationale.com/sr-71timeline.pdf