The idea of an electric harmonica goes back to the 1920s. Pat Missin created a very informative page about this topic. He also shows a picture of the Hohner electronic harmonica prototype from the late 1970s. The prototype still exists. I had the fortune to take a picture of it at the harmonica museum in Trossingen, Germany. Unfortunately, nobody at Hohner can provide any information about its history. On top of that, the connector cable is nowhere to be found. Without that cable it is not possibility to get it to work. I would have loved to hear how it sounds.
The first really useful electric/midi harmonica was the Millioniser 2000, invented by Walt Miller. You can find the full story story about it online. It was quite an advanced technology for its time.
Update: Lekholm instruments just recently released an updated version of the DM48.
The new model, DM48x has will add the following new features:
-Wireless playing over Bluetooth Low Energy (BLE) MIDI
-Ribbon controller: pitch bending or modulation via finger control
-Built-in battery, for five hours of wireless playing
-more onboard memory
In April 2021 a new MIDI digital harmonica by the Chinese company Mozka Digital was released.
Three models, HM12s, HM12 standard and HM12pro are available varying in features. All come with 78 onboard sounds. The instrument can also be used as MIDI controller for external synths.
It features wireless playing via Bluetooth. Some more info (mostly in Chinese) can be found here.
Brendan Power has a demo and review video about it on YouTube.