You could do a search on Pro Air on this forum and find a bunch of posts about them and why they went under, but I'll save you the trouble.
Pro Air (P9/PRH)was headquartered in SEA but had their hub at Detroit City Airport, flying a fleet of 3 737-400 and 1 737-300. They started out flying to BWI, EWR, IND, MKE and PHL. They dropped MKE, but added LGA, MDW,ATL and seasonal service to MCO, TPA, and RSW if I'm not mistaken. They then added a flight to SEA and made MCO a year-round destination. All this while keeping a 4 plane fleet. Pro Air also had an agreement to fly GM workers at a discounted price, and planned forming a regional branch called Pro Air Express flying Saab 2000's to BUF, GRR, and a few other places I can't remember. It never happened though. P9 still hemorraging money and dropped EWR, IND, PHL, and SEA and went down to flying 3 -400's.
After this they had a ton of operational problems, frequent cancellations and horrendously long delays, and many times ran their route network with just one of the three planes because the other two had some kind of maintenance problem. During the summer of '99, P9 was heavily scrutinized by the FAA for maintenance issues including flying unairworthy aircraft and failure to maintain proper records of maint. work. I saw this for myself many times while working P9 flights at MDW. For example, one of the planes flew around for weeks with a large dent in the leading edge of the wing. I believe it was N460PR, the red tail. Anyway, the FAA ended pulling P9's operating certificate. Pro Air appealed but lost and ended filing for Chapter 11 shortly thereafter, having taken a loss of $52 million in just three years.
Pro Air is now getting its act together with the FAA and is planning to make a comeback in Q1 of 2001. There headquarters will be moved to Detroit and I'm guessing that's where they'll keep the hub. They had a pretty good niche going, but there were a few factors other than the FAA that brought this airline down. Hopefully they'll have better luck if they can make a second run at it.