I seem to recall installing fans 20 years ago that hung from the box. All the fans I've installed within the last 5-6 years have a bracket that screws to the joists and the fan hangs from the bracket. The ceiling box provides no support for the fan. A fan rated box is required only if the fan hangs directly from it (sole support).
314.27
(D) Boxes at Ceiling-Suspended (Paddle) Fan Outlets.
Outlet boxes or outlet box systems used as the sole support
of a ceiling-suspended (paddle) fan shall be listed, shall be
marked by their manufacturer as suitable for this purpose,
and shall not support ceiling-suspended (paddle) fans that
weigh more than 32 kg (70 lb). For outlet boxes or outlet
box systems designed to support ceiling-suspended
(paddle) fans that weigh more than 16 kg (35 lb), the required
marking shall include the maximum weight to be
supported.
These fans need some serious support:
http://www.bigassfans.com/