From their announcement:
What makes the Mobile Airfields so recognisable are their inventive and distinctive designs. Perhaps more impressive is their sheer size and presence. Best of all, you can now add these beastly war machines to your collection!
The wait is finally over for those of you wanting to buy the remaining Sky Fortresses and Carriers. Now you have the choice of fielding a Sky Fortress or a Carrier. Perhaps you will field both. It would certainly bring to the table a battle of exciting and epic proportions!