The drone was called the D-21, and it was not a variant to the SR-71, or A-12 as the original design and proposed fighter/interceptor was called, but a small drone that rode piggy-back on a SR-71 that was launched from an aircraft carrier.
The drone was flown on three sortie over the China and Siberia mainland. The first sortie flew about 300 miles inland and then disappeared from the scope. The next drone flew the entire route, but crashed into the water before it could release its film canister. The final drone flew perfectly and released its film canister correctly, but the c-130 that was supposed to receive it missed the catch and so the project was scrapped.
Also, the SR-71 was painted with radar absorbing paints. The combination of these paints and small design changes, like the stealth, made the SR-71 the first stealth airplane.
The spy plane was originally retired in the early nineties, but in 1994 it re-entered service as a spy plane for a short time. Now the only spy plane governmentally acknowledged still in service is the updated version of the U-2. The reason given for the retirement of the plane was that it is too expensive to maintain and the uses for it were not very important. Some sources near Area 51 say that there is a new plane that has not been revealed to the public, named the Aurora. Now most aerial spying is done by Unmanned Aerial Vehicles, or UAVs.