Thanks for the input, everybody.
In other states I am familiar with, concrete encasement of steel stringer bridges, sometimes even just the fascia beams, was not uncommon in the 1920s and 1930s. Without the view underneath, my vote is that may be what is going on here.
The Arkansas City vicinity has several small concrete bridges from the early 1900s. They tend to cross small "canals" in the town. I am going to try to sort out the bridges and add them to bridgehunter.
Note: For those not from the area, the town name is pronounced arKANSAS City, not ARkansaw City. Most locals call it "Ark City."
I strongly agree with Clark; you will never know for sure without a look underneath...also, I would venture to say, looking at the thickness of the portions on the side of the bridge under the railings, that the NBI is probably correct...if this were a slab of that thickness, it would be so heavy that no substructure would ever be able to support it; these portions are most likely concrete facades that hide the stringers. The railings seem to be sitting on top of this portion rather than being part of the superstructure as a whole, possibly eliminating the through-girder option as well. It looks to me like the railings do not contribute to the structural strength of the bridge...I would bet that it is indeed a steel stringer, or possibly a concrete T-beam bridge...
Through girder, slab, beam, stringer--it's like determining the gender of a rabbit--you need to take a look underneath.
I need to look in my vintage concrete design books to see if there's an easy way to determine visually whether the railing contributes structurally. I don't recall seeing anything.
It's very nice to see this classic small town bridge from the early days of "modern" roads.
I will be uploading photographs of this bridge in the near future. For now, I have added Street View.
Although NBI classifies this bridge as a steel stringer, I believe that it is a concrete through girder. As always, any feedback is welcome.