SR-71 Blackbird
     Successfully  complete  the  Pilot  bonus objective for the final
mission (Mission 4.5: Final Count Down) to unlock the SR-71 Blackbird.

The Big One (SR-71 Blackbird Weapon Pack)
     Successfully  complete  the  Ace  bonus  objective  for the final
mission   (Mission   4.5:  Final  Count  Down)  to  unlock  the  SR-71
Blackbird's  fourth  weapon pack. This weapon pack includes the game's
super weapon, the A-SLAMR. The A-SLAMR does not recharge once fired so
use it wisely. Note: The SR-71 Blackbird and "The Big One" weapon pack
can both be unlocked at the same time if playing on the Ace difficulty
setting. Just complete both of the required bonus objectives to unlock
the  SR-71  and  "The  Big  One"  weapon  pack  together  (instead  of
separately).