I've never actually confirmed if it is true or not, but I read up on some details about Fantasy of Flight and the airfield several years ago, and one of the items mentioned was that below the grass surface of the runways, is actually hard-surface concrete runways. This allows for the WWI aircraft in the collection with tail skids that need to be flown off grass surfaces to be operated properly from the airfield, and with the hardened surface below, allows for warbirds as big as B-17's, B-24's (like the collection's B-24 "Joe"), and as the write-up mentioned, even a B-29 (which would be the largest land-based aircraft in the museum, if one of the examples owned by FOF, in storage, were to ever be made airworthy), to be flown on/off the main runway safely. Of course this is something I read about, a number of years ago, so I don't know how true that is. For those not familiar with the airfield/museum will find, the runway is certainly long enough for large warbird operations (with FOF owning a B-26 Marauder that operated from the airfield until it was no longer flown, the collection's B-25, which is coming back up to flight status at the moment, and a few different DC-3's/C-47's have flown in and out of the airfield, including the collection's recently obtained C-47). Kermit Weeks has also spoken about some of the items he still hopes to build as part of the Fantasy of Flight experience, including a seaplane hangar and ramp, similar to those that could be found in the 30's when the great "Clippers" were in operation, where the collection's seaplanes, including the Sunderland, Sikorsky S-39, Duck, and others would be housed.
If you're on Facebook, both the Fantasy of Flight and Kermit Weeks pages are great to follow, with daily (sometimes hourly) posts and updates on what's happening.
These are some photos just posted to the FOF Facebook page today, and this all happened at Fantasy of Flight just this morning! (Photos are all courtesy of Fantasy of Flight)
(Note that with Dimus' amazing attention to detail, all of the various aspect of these photos can be transposed with that of screenshots and in-game views of his work.)