I have personally always been of the opinion that it would make sense for the default integer type to be unsigned, though it's been a long time since that would've been a live issue for debate; C in the 1970s was already defaulting to signed integers, nor was it the first language to do so.
I'm interested in exactly when, why and how the decision was first made. In assembly language, there isn't really a default; you always specify signed versus unsigned. So we should look at high-level languages (using here the classical definition of high-level as 'higher level than assembly'). The first significantly influential high-level language was Fortran. Modern Fortran standards mandate that compilers shall treat integer variables as signed unless otherwise specified.
When did Fortran decide this? Was the decision already made in the earliest Fortran compilers? Did any compilers on any machines, treat integers as unsigned?