The first STOVL F-35B, aircraft BF-1, returned to the air with a hour-long flight on Friday, restarting the countdown to the first vertical landing - in October would be my guess. BF-1 was down for an extended period of modifications resulting from previous flight and hover-pit tests, and proved reluctant to return to flight.
There were several wiring issues, which were finally fixed by rerigging the weapon-bay doors, then the integrated power package proved troublesome, and was eventually replaced with a redesigned unit already flying successfully in BF-2. Now that it's back in the air, BF-1 is scheduled to complete a series of flights with STOVL doors open, and lift fan engaged and disengaged, to check flight-control software changes made as a result of the first series of doors-open flights.
Some time this month (ie September), BF-1 and BF-2 are scheduled to be ferried to the US Navy's test center at NAS Patuxent River in Maryland, where BF-1 will complete STOVL build-down flight testing to a vertical landing. The ferry flight will be a significant event as it will mark the first delivery of development jets to a service test center.
BF-2, the second STOVL aircraft, has been down for final finishing (aka painting) and is due on the hover pit this week as a prelude to returning to flight. BF-3 is in extended ground testing, so the next F-35s scheduled to fly are the first production-representative CTOL bird, AF-1, and the final STOVL test jet, BF-4.
And now, just for fun, here's a recent LockMart flight-test video which includes a segment showing BF-1 during its earlier doors-open, lift-fan disengaged flights...
Video: Lockheed Martin
And here's a brief clip from Lockheed Martin's second-quarter program update video in which F-35B chief test pilot Graham Tomlinson discusses BF-1's hover pit tests...
Video: Lockheed Martin