A lot of bikes are good candidates for touring, even if they're not marketed as touring bikes. Here's what I'd look for, in no particular order.
- Comfortable fit
- Solid construction
- Rack or fender braze-ons or both (If I really liked a bike without braze-ons, I might get it anyhow and add a small trailer.)
- No suspension system, front or rear (For touring a lot on rough roads and trails, I might consider getting something with a fairly stiff front suspension but no rear suspension.)
- A wide range of gearing, especially for touring in the mountains
I've toured on mountain bikes and on road bikes. Only one bike I've used for touring was designed for that kind of riding. One of my bikes was designed for off-road racing while another was made for running about in the city. They all worked well for touring.