The VT proximity fuse was one of the biggest cost/rush programs in the US in WWII. 

The first costliest program was the B-29 bomber program, at about $3 billion in 1945 dollars. Harry Truman ferreted out as much as he could, but the whole project w as a huge rush job as the "bomber mafia" was found to be rather wrong about the viability of bomber-only missions over Europe.

The second costliest program for the US was the Norden bombsight, at over $2 billion in 1945 dollars.

The third costliest program everyone knows about now as the Manhattan Project, at a wee bit under $2 billion (in 1945 dollars), but which Harry Truman, leader of the Truman Commission until becoming VP in 1945, could "not find out where all that  money was going to in New Mexico."

The  fourth costliest program was the VT proximity fuse project, started in 1940. It was in use in the Pacific over water (so that duds could not be retrieved) in early 1943 and onwards, and it proved itself from the get-go. Suddenly, 5" guns on US Navy ships were turned into aircraft killing machines. This project was over $1 billion in 1945 dollars, and it had as much priority on men and talent as the B-29 or atomic bomb program. Men who were engineers who had enlisted for front-line service could suddenly find themselves with orders to report to a lab in the US, no choice in the matter.

There was one limited application over friendly land in 1944, when Hitler launched the "V-1" Buzz Bomb campaign against England in July of 1944. Churchill got the VT fuse turned loose by Allied Command and by the fourth week of the V-1 campaign in August 1944, the Brits were bagging over 75% of V-1 in mid-air.

The VT fuse is a wonder of engineering. The radio transmitter/receiver inside the fuse have to withstand an acceleration of 20K G's, spin of over 500 RPM, and be able to be handled safely in transport. What is most remarkable is that all the electronics are based on vacuum tubes, which aren't exactly known for handling rough conditions.

Come December 16th-17th, 1944, when our forces were looking like they might be overrun at the edge of the Ardennes, the VT fuse was authorized for use in field artillery, and the fuse, combined with the US Army's incredible talent for "time on target" fire-for-effect attacks, slaughtered Germans in their foxholes, in the open, in forests,  you name it. I've seen German accounts (in German) of what it was like to be German infantry when we started the TOT+VT fire missions, and the German morale collapsed rapidly - the very few, very lucky Germans who survived would look around them and see everyone in sight but themselves torn to shreds. There was nowhere to run, nowhere to hide, no way to hear the shells coming in and take cover.

And as you say Ed, it was highly, highly secret. Many people working on the VT project had no idea what they were working on, because they were working on only one small component or issue in the project and had no idea what the overall project was.