This is what Wikipedia says near the end of its radio altimeter article:
The radio altimeter first showed up in the German Junkers Ju-87 "Stuka" dive bomber which was equipped with one for automatic pullouts in the dive bomb run which usually consisted of an 80-90 degree dive. The Stuka pilot would set the radio altimeter to 750m which was connected to the bomb release and automatic pull out. The radio altimeter would drop the bombs and pull the Stuka out of the dive at the set altitude to a level flight. This was invented because the pilots would black out on the pull out, usually for 2 to 5 seconds.
No citation, and I would be skeptical anyway. I know the Stuka had an autopilot that pulled out of a dive, but I thought it was activated manually by the pilot dropping his bomb. I figured he would just drop his bomb at whatever altitude he judged by his own eyes, rather than watching some altimeter instead of the target.
I'll also note that Wikipedia is not self-consistent here. If you scroll up just a few paragraphs, it says:
In 1924, American engineer Lloyd Espenschied invented the radio altimeter. In 1938, Bell Labs put Espenschied's device in a form that was adaptable for aircraft use. In 1938 in co-operation with Bell Labs, United Air Lines fitted a radar type device to some of its airliners as a terrain avoidance device.
Two citations there. So I don't know what the author means exactly when saying it "first showed up" in the Stuka.
I would have thought such a radio altimeter would have been very difficult without the cavity magnetron, invented in Britain and sent to America in the Tizzard Mission of 1940 September.
On the other hand, I have come across some night fighter radars that Germany had, such as the Lichtenstein Radar that did not need cavity magnetrons. That was not used until 1942, though.
So did the Stuka really have some radio altimeter? If so, when? Was it really the B version that saw action in 1939? What frequency and power did it use? How big was it?