I have looked up how to make Banana juice because I wondered if it was possible. It turns out that it is.  I read that it is best to use a blender for this.  Wouldn't I just end up with a banana puree if I use a blender to try to juice it? 
I've made banana juice a number of times. And I'm referring to a relatively clear liquid extracted from the bananas. It's a pretty straightforward process that produces a high yield. The water in bananas is chemically bound to the starch in bananas and this liquid can be released by breaking down the starch using an enzyme called amylase that is present in bananas. Here's how I do it:  (Yield 1 liter of juice)   Take 2 kilograms of bananas, peel and puree them. Pour the puree into a vacuum bag and seal. Alternatively, you can put them in a VERY securely tied plastic bag with the air pressed out or you can put the puree into a canning jar. Next place this in a pot of water at around 150 Fahrenheit/65.5 Celcius and let sit for 5 hours. This allows the naturally occurring amylase in the banana to fully breakdown the starch in the banana and release the liquid.  Strain the liquid through a nut milk bag (my preferred method), or you can use several layers of cheesecloth, but a nice nut milk bag makes this so much easier. Squeeze (and massage to get the highest yield) the juice through the bag. You should get about a liter (generally a bit more than that) of fresh, delicious banana juice!  