The term "kitniot" is sometimes translated with "legumes" or "beans". The actual translation might be different (something like "small things" according to Balashon), but this is an indication of the importance of beans for the category. Basically every kitniot list contains beans, see e.g., OU, Kashrut.com, Star-k, …
But what are "beans"? A bean is described in Wikipedia as "an edible legume" (Wikipedia:Bean) and a near-synonym of pulse which in turn is defined as "an annual leguminous crop yielding from one to twelve seeds of variable size, shape, and color within a pod" (Wikipedia:Pulse). So now – what is a legume? "A legume is a plant in the family Fabaceae (or Leguminosae), or the fruit or seed of such a plant" (Wikipedia:Legume). So back to beans: The Wikipedia article contains a list of bean types, all of them are part of the subfamily Faboideae.
Confused? Yes. Me too. As I am no botanist, let’s get back to the supermarket-definition of beans. Most people if you say "bean" will probably think of something like this:
Green beans, photo (c) Rainer Zenz
These are varieties of "phaseolus vulgaris" (or common bean) from the "phaseolus" type. There are other types of beans listed in the Wikipedia article that (to me as a non-botanist) are very similar, e.g., Vicia (includes fava beans) or "Vigna" (includes Azuki beans). For this discussion, let us assume that the halachic term "beans" refers to all of these.
Beans have been on the kitniot list for a long time. Historically, beans grew near to grain or fields were alternately used for grain and legumes. This crop-rotation has been practised in Europe since the 8th centory and works as follows: "Under three-field rotation, the land was divided into three parts. One section was planted in the autumn with winter wheat or rye. The next spring, the second field was planted with other crops such as peas, lentils, or beans and the third field was left fallow. The three fields were rotated in this manner so that every three years, a field would rest and be fallow." (Wikipedia:Crop rotation) So the fear of some actual chametz grains mixed in with the beans was very real at that time. That crop rotation is a good idea seems to be recognized today, but I do not know what percentage of beans we can buy in the supermarket grows on such fields.
Dried beans can be ground into flour which could be mistaken for chametz flour. I have found an article quoting the History of Bread: "In addition to wheat and rye, bread was made from oats, barley, grass seeds and in times of shortage and famine, even peas and beans!" (Kosher.org.uk). I am unsure if people today still use bean flour, I have definitely never seen it. I cannot think of cooked bean dishes that look like chametz dishes, but maybe I just do not eat them. The recipes I know with beans always contain very distinguishable beans.
The above concerns are about the seeds. What about the fresh green beans (string beans)? The situation doesn’t seem clear. There is an opinion by Rav Lior that "Legumes such as green beans and peas which are still in their pods are considered vegetables and not Kitniot." (source). On the other had, apparently green beans and peas are considered kitniot in the Sefer She’arim HaMetzuyanim BeHalacha, Siman 117, Si’if Katan 7 (I don’t understand it, but read the discussion about what the text is actually saying here). The Star-k list contains "String Beans", "Green beans" and "Bean Sprouts". The Kashrut.com list contains "String Beans", while the OU list contains only generic "beans".
- Danger of chametz traces: Yes (crop rotation).
- Danger of confusion with chametz (raw): No.
- Danger of confusion with chametz (processed): Yes, as flour.
- Botanical categorization: Legume.
- Known in 13th century: Yes.
- Verdict: Seeds are probably kitniot, green string beans are probably not.
Edit on March 24: Fixed formatting.