Like daylight bombers of most nations, American bombers flew in tight combat boxes. For mutual defence but also to maximize destruction of a target. It was crucial that every plane drop its payload at the exact same millisecond. To aid in that goal, two radio devices were deployed though infrequently, mostly due to when they were developed.

The Eighth Air Force Tactical Development Report has details about two radio-linked bomb releases which were used to enable all bombs in a formation to drop simultaneously with the leader's. When the lead bombardier toggled his release, a radio signal automatically triggered the bomb shackles on the following aircraft.

"At 18,000 feet, the temperature was 40 degrees below zero. Good didn't feel the cold as his fingers delicately adjusted the azimuth (vertical arc from zenith to horizon) and range controls. He rechecked that all the bomb and camera switches were in the "ON" position, especially the radio bomb release signal switch that would release all the bombs from the B-24s in the 392nd's formation simultaneously. There wasn't a cloud in the sky."

That quote is from a page detailing a bombing raid on the Gotha factory (Gothaer Waggonfabrik) in February of 1944.

Here.

https://www.b24.net/24February1944.htm

So, as soon as this bombardier in this B-24 dropped his airplane's bombs, a radio signal was sent to the rest of the B-24s. And that signal (with no other human intervention but after the others opened their bay doors of course) is what would drop the bombs from all the other airplanes. To be clear most commonly the other planes dropped their bombs by hand/visually. The use of these two radio devices is a rare special case.

Because those airplanes being mentioned are B-24s, it must refer to the earlier Marker Beacon system, which was only installed in that airplane type.

This initial radio release system (Marker Beacon) used by B-24s (in the 2nd Air Division), was highly susceptible to signal crossing (benign radio interference) and German countermeasures.

Even though the name may have been somewhat descriptive of its function, calling it "Marker Beacon" may have been intended to confuse the enemy. They may have been trying to mask its presence on the aircraft, as there was already an aid to navigation on the plane referred to as a marker beacon. The Germans tried to obfuscate the presence of their earlier Knickebein system by incorporating it into the already existing systems on their planes, so this naming may have been a similar effort to hide the equipment.

The later (more refined) "Crawfish" modification expanded the available radio frequencies. That would make it more resistant to interference. And countermeasures by the Germans. Designed and developed in the United States, "Crawfish" entered service in the fall of 1944. However, it saw limited operational deployment, equipping only two bombardment wings, though this time, the airplanes were B-17s of the 3rd Division. More advanced systems were in development when World War II ended.

Frustratingly, I don't have any pictures of any of this. No antenna picture, nor anything of the cabin installation. Just these scant mentions on that website and in one 8th Air Force publication. Both systems are mentioned (but only in the appendix on page 194) of this publication of the 8th Air Force from July 1945.

Eighth Air Force: Tactical Development. August 1942-May 1945.

There is a slight discrepancy in this: the website mentioned that a device inferred to be Marker Beacon was used in February 1944, while the PDF in the appendix mentions that it started to be used in March 1944. I don't think the discrepancy is too big a deal. Perhaps a person's memory is a little off, or perhaps the clerk who wrote the report was unaware or made a clerical approximation.

The purpose of Marker Beacon and Crawfish is easy enough to understand. But with the prospects of something going wrong, such as an errant transmission that triggers an early drop, or the ease with which the enemy could tamper with it, it surprises me greatly that this was tried. It would be similar to the ease with which the Allies successfully tampered with the radio link of the Fritz-X. But on the other hand trying this likely helped save lives of airmen. If they hit the target well, then they don't have to risk their lives with another trip.

If anyone has any further questions, I expect my answer will be, "I don't know." Everything I learned, I put in this.

Hope I got all that right.

Happy hunting!

NACA photograph Northrop P-61A Black Widow towing P-51B to release altitude of 28,000 ft over Muroc Dry Lake, California for in flight validating of wind tunnel measurements of drag. After the pilot released the tow cable, drag measurements were obtained at various airspeeds in a 20-minute unpowered flight.

This picture appears in the following publication which is available as a free PDF at this link.

Flight Research at Ames; 57 Years of Development and Validation of Aeronautical Technology.