In this link above, there's one of the best (my opinion) and earliest videos of O Sensei from 1935 shot by Asahi News. The full footage is available from Aikido Journal Online, and I recommend getting it to see the deleted bits. In this clip, around the 6:25 mark he shows a bunch of Juken techniques - Japanese rifle and bayonet. These techniques are pretty much what I learned for Jo Dori, and most of the Jo Dori I learned could be applied to the extra long rifle that is used here. Within the last few centuries, availability of bullets depended heavily on logistics for the larger army and sometimes a soldier didn't have much ammunition on them. The rifles also had a smaller magazine compared to today, and reloading was needed more often. A rifle became a combination spear and club for close quarters combat.
O Sensei's one book, the only book in which he posed for pictures, is Budo. This was published in 1938, and the pictures are from Noma dojo. Alongside the empty hand techniques are three Tanto techniques (which the text said are for use against pistols or knife thrusts), 4 bayonet techniques with a fifth technique demonstrated against a spear (while the text makes the point it is the same as against a bayonet), and eight sword techniques. In other words, the Jo is not shown and the rifle and bayonet gets near equal focus and attention as the sword. The sword and the firearm techniques get the same weight overall.
Several authors refer to O Sensei having very high skill with a rifle and bayonet, and he is known to have studied the spear and the sword. His life is very well documented, and he had certificates from several martial arts. There's one certificate that is absent. The Jo is a very famous Aikido weapon, and associated with Aikido moreso than most other martial arts, but there is apparently no documented history of O Sensei studying the Jo.
I meet many Aikido students who don't even know the story of Muso Gonnosuke and his two duels with Musashi. Some versions refer to Gonnusuke as a master of the Bo (6 foot) staff who was defeated by Musashi. The story goes that Musashi was impressed with Gonnosuke and they did become good friends according to Yoshikawa's fiction (Yoshikawa writes that the Jo was already something Gonnosuke was already using, not a weapon developed as a result of the loss. Other variations of the story say the opposite.) Gonnosuke was beaten and disarmed by Musashi in their first duel and Musashi spared his life. I'm not sure if that would have been an insult. To take someone out without hurting them does require that you be much, much better than they are.
Gonnosuke meditated and trained, and again by Yoshikawa's fiction he was in close contact with Musashi and influenced by Musashi's approach to training. He developed new techniques, new kata and (in some versions of the story) a new weapon. The Jo is such a simple weapon that presumably it actually predates any sword or spear techniques, but Shindo Muso Ryu was Gonnosuke's creation and it survives to this day. His exact inspiration is unclear to me, though various legends are told frequently (just never the same story twice.) Some say that Gonnosuke developed the Jo to be longer than the sword, but the Bo staff was already longer. The short staff could be more accurate when used to stab. The longer spear techniques are better suited to formation fighting or open battlefield movements, but it would be problematic in a crowded environment. Musashi was famous for using the terrain against his opponent, and I can imagine a duel in a heavily forested area that would leave Musashi the advantage, but I can't prove it.
Another anecdotal thing I was told by a student of Shindo Muso Ryu was that the larger polearm weapons could break. So, a master of the spear or Naginata (A full length staff with a sword on the end) would then learn the Bo (6'), the Jo (4'), the Hanbo (3') and Yawara (6" more or less).
So, the Jodo and Jojutsu existed for centuries in more than one form centuries before O Sensei. These were established Ryu, and presumably to study in one system of sword or spear was to be exposed to some Jo. Saito Sensei's creation does differ from Shindo Muso Ryu in a number of details.
I was taught the 12 basic kata of Shindo Muso Ryu. This link immediately above shows the ready position that Jojustsu uses: The Jo in the right hand, with the front end aimed at the eyes of the opponent. I was told that Uke should not be able to see how long your weapon is as thay can only see the very tip. From here, Nage steps forward and directly strikes.
The Aiki Jo stance is very different. The Jo is held in the left hand, held vertically displaying it's full length in something very like a Kyudo posture. A number of modern militaries will have the rifle in the left hand with the butt on the ground while standing at attention. The left hand raises, and the right goes to the trigger. Much of the Kumi Jo would be very effective with a rifle and bayonet. The videos below are from Saito Sensei, who is the author of these forms and not O Sensei. I had been taught the Iwama Kumijo with the right hand going below the left, to a point around where a trigger would be on a rifle. Saito Sensei often seems to grab the top of the staff and shows the first movement being done with the butt end.
In post war Japan, when Aiki Jo really appeared for the first time, the American forces had occupied Japan and many martial arts schools were ordered closed. I can only assume that the Jo was a much more innocuous looking weapon that allowed the Founder to do what he already knew, and he could then work to develop his weapon further from there as he no longer had to pay attention to moving parts, triggers and bullets. He could be free to hold the weapon at any spot along it's length. Shioda Sensei had one of the earliest clips I had seen of an Aikido master using a Jo, and he was demonstrating to a group of policemen. The police riot baton in Japan might also be an influence on our Jo work, though police jo work is more closely resembling Shindo Muso Ryu.
The enlightenment of Muso Gonnosuke might have been some inspiration, but the Aiki Jo and Shindo Muso Ryu are different in that the commonly used holding each end of the Jo in Shindo Muso Ryu would be crazy and dangerous to do with a rifle and the right handed grip in the middle of the weapon would not be practical for a firearm either. Much of Aiki Jo can easily accomodate a rifle, some movements with more modification needed than others. The movements that have the Jo pulled back to a "resting position" would be crippling or lethal if done with a blade and they would bring a rifle to a useful position or bring the barrel back towards the target. With a staff, the movement is much more difficult to apply.
The later developments by Chiba Sensei and Nishio Sensei were apparently influenced by their other studies of existing Japanese martial arts, giving each of these styles different flavours and appearances.
Times have changed - close quarters combat in an environment like Iraq today involves very good logistics and tight spaces. Firearms are shorter than the 1935 video shows, and they carry more bullets now. In an urban environment, you can move more freely with a more compact weapon. A long barreled rifle would be impractical for storming a house.
I think that rifles are the forgotten Aikido weapon. Philosophies changed, wars were lost, combat changed and so did our practice of today. Jukendo might have Jojutsu and Jodo as an ancestor, but I believe that O Sensei's development of Jo techniques was influenced by his extensive experience with firearms.
From Stanley Pranin, this is one of his entries on Iwama Aikido when people started to discuss Aiki Jo and it's history. The thread is there for people who want to read the whole thing, and some of it is good and some of it is the ramblings of crazy people who offer little information. Mr Pranin is right that there is a huge amount that can't be substantiated, and that leaves a vacuum. When O Sensei's own son and his longest direct serving uchideschi have no idea where Aiki Jo came from, I have to abandon hope of a clear solid answer.
I would like to inject something here. Whether or not you realize it, you are taking the rather formalized system of Morihiro Saito Sensei as your point of departure for understanding aiki jo. What O-Sensei left of the jo and the ken, for that matter, were fragments and practice routines, not much of a formalized nature. It was Saito Sensei who systematized these movements into the forms that we consider aiki ken and jo today.
I'm sure there were mutliple influences that factored into Morihei's practice of these weapons. As a historian, I tend to emphasize what can be specifically shown to have occurred rather than rely on speculations and anecdotal information obtained from parties other than principals involved.
One also needs to keep in mind that O-Sensei's life is very well documented. So the fact that no hard information exists on a certain possible influence tends to support the thesis that if any influence did exist, it was minor and of a transient nature.
O-Sensei was very good at observing and absorbing what he saw into his own practice. Also, he was exposed to many skilled martial arts practitioners through demonstrations and a network of acquaintances throughout his career. There was ample opportunity to have seen a great number of weapons systems and have adopted certain elements into his own practice and experimentation...
...At various times during my stay in Japan I attempted to try to pin down the origins of the aiki jo. Neither the Second Doshu Kisshomaru Ueshiba nor Morihiro Saito had any idea. I asked Sadateru Arikawa Sensei but he was not sure either. In fact I asked him about a possible connection with Kukishin Ryu and he made a funny face. Apparently, Toshitsugu Takamatsu had met the Founder but their acquaintance was superficial according to Arikawa. The photo that shows Takamatsu posing with his arm draped over Morihei Ueshiba's shoulder is really an odd document. Since Ueshiba was an older person of considerable status, this according to my sensibilities borders almost on rudeness. I get uncomfortable just looking at it.
I think that Jakob Blomquist's perspective is quite likely correct. The Founder was very good at observing, absorbing and adapting what he saw at demonstrations and training sessions. He had extensive contact with many of the martial arts masters of his day.
As Ellis points out, we know about the connection with Kashima Shinto-ryu because Meik Skoss noticed the similarity of movements of the 1st and 2nd kata with those of the aiki ken. This was further confirmed because of the fact the Morihei Ueshiba actually enrolled in this art and took a blood oath. Also, teachers from this classical school taught at Ueshiba's Kobukan Dojo for a year or two starting in 1937. This has all been thoroughly documented.
What O-Sensei left of the aiki jo was mainly fragmentary movements. Much of the systemization that became what are known as the Iwama Aiki Ken and Jo was done by Morihiro Saito Sensei, not the Founder. Saito Sensei was a very methodical person, totally unlike the Founder. Saito was also a very accomplished teacher in a western sense while O-Sensei was spontaneous and impulsive in his approach to teaching. The aiki jo is likely an amalgamation of many different forms of input that Ueshiba picked up from observing numerous experts. He then did a great deal of experimentation particularly during the Iwama years (1942-1955) and left numerous movements and partial sequences that were later formalized by Saito Sensei.
The first most obvious question is does it really matter? Ultimately, a jo is a stick with two identical ends. In my understanding, in Europe, sticks were for practice for people who couldn't be trusted with edged weapons or for fighters who couldn't afford or obtain the spear or halberd tips. It was a cheap, easily obtained weapon that was safer to practice with that people could use, or use to prepare for other weapons with. While a traditional Ryu did exist, this is a weapon that could date back to the caveman.
If O Sensei could "filch" technique by just watching someone, then the assertions that he learned some Baguazhang or other arts in China start to get slightly plausible - not as a formal student, but as someone who got attacked frequently and eventually arrested. He is known to have had several acquaintances in China during the war years. I just don't see Aiki-Jo bearing a resemblance to most Wushu staff work. https://m.youtube.com/watch?v=BvUa99vHKI4
So the real question I suppose is what we do next if we don't know where we've come from. Does it matter to our practice? Do we allow Aikido weapons to evolve? We have freedom and invention in our taijutsu, but our weapons in most styles are the most rigid aspect of Aikido training. That doesn't look to be the case with O Sensei's practice. The innovations of Chiba Sensei, Saito Sensei, Shirata Sensei and Nishio Sensei are all influenced by the Founder and all very different and maybe all equally valid.
When I watch the clip below, O Sensei could have been holding a Jo in all of his movements without much modification. Whether or not his Uke are armed or there is more than one attacker, he changes very little. His Jo demonstration looks spontaneous and completely free. I saw something similar in the Shioda Sensei clip I posted here too. The weapon becomes incidental, and there is no need to have a separate practice or a completely different way of moving because one picks up a stick.
I'm sorry I wasn't able to get this videos imbedded or cut to the one part I wanted to refer to - but they're all worthy.