I am so glad you and your friend escaped without injury, Sig. Sounds like a harrowing experience--my heart was beating hard as I read it!
My brother was attacked by a pit bull (he escaped but it tore a hole in his jacket) and my wife's nephew was not so lucky--the dog chewed off and ATE the skin on his wrist, and he lost a lot of blood and had to be flown in a helicopter to the hospital, where they had to graft skin from his thigh onto his wrist. This dog had been chained up in the front yard but the threaded link had worked its way loose as the dog pulled on it in his frenzy. Nonetheless, the owner was found negligent and had to pay damages and the dog was destroyed.
I love dogs but I am mistrustful around all breeds until I get to know them. They are animals and I have to find out if they have been properly socialized before I will interact with a strange dog. As a former paperboy who has been bitten by several different breeds, I have a built-in wariness of dogs until I get to know them. Ironically the ones that have bitten me the most have been the small breeds. But what makes pit bulls dangerous is their incredible jaw stength, they simply do not let go once they've decided they're going to hold you.
While it's certainly smart to get to know a dog before you trust it, the belief that pit bulls have very strong and/or lockable jaws is extremely common but false. Jaw strength in any dog is largely related to size; pit bull jaw strength has been measured around 235 psi, a little less than a German Shepherd and far less than a Rottweiler. Of course it's still strong enough to cause significant damage, and as I said before, it is always smart to be wary of dogs you don't know (especially chained up dogs; a dog that is chained up and ignored is a bite waiting to happen).