According to this Flyertalk thread, Chase is offering 50,000 miles as a signup bonus (after $250 in spend) for a Mileage Plus Visa.
The offer isn’t available online, you have to call. From the thread,
Call Mileage Plus Customer Service (1-800-421-4655 for non-elites)
1. Choose “manage my account”
2. Let them know your mileage plus number and zip code
3. Say “more options”
4. Finally say “apply for a United mileage plus card”.
And tell the rep that you’re calling for the 50,000 mile signup bonus offer.
All reports on-thread so far are positive.