In 1942 scientists in Britain developed an idea that they believed would confuse Germany's radar system. Given the codename of Window the strategy involved the Pathfinder Force dropping strips of metallised paper over the intended target. By early 1943 a series of tests had shown Bomber Command that Window would be highly successful. However, the British government feared that once the secret was out, the Germans would use it to jam Britain's radar system. It was not until July 1943 that permission was finally given to use Window during the bombing of Hamburg.
Window was a great success and was employed by the RAF for the rest of the war. The Germans were forced to change its strategy in dealing with bombing raids. As Air Marshall Arthur Harris later pointed out: "The Observer Corps now plotted the main bomber stream and orders were broadcast to large numbers of fighters with a running commentary giving the height, direction and whereabouts of the bomber stream, and of the probable target for which it was making or the actual target which it was attacking."
The main objection to the use of "Window" (the strips of metallised paper) which proved to be the most important and effective of all the weapons used against enemy radar, continued to be the fear of its effect on our own defences. It was hoped that our own radar would be developed to the point where the strips
of paper would not cause any very serious interference, but even so, defensive radar might never be quite so effective after its introduction as before. When I continually pressed for the introduction of this weapon, other objections were also made. It appeared that we were short of suitable plant for the manufacture of the strips in quantity, and that it would be very difficult to get priority for the supply of aluminum needed. There can be little doubt that if we had been able and allowed to use this weapon in the first months of 1943 we should have saved hundreds of aircraft and thousands of lives and would have much increased the accuracy of our bombing.
There was every reason to believe that if the authorities would only allow us to drop strips of metallised paper during our attacks we should hopelessly confuse the enemy's radar on which he relied for the control of his night fighters and the accuracy of his gunfire. Early in 1943 there had already been developed a suitable form of this weapon for jamming the enemy's ground control stations, radar-sighted guns, and airborne radar for interception. And we had already worked out the quantity of strips of paper that would be required, the rate at which it should be dropped, and the areas over which it should be released. It cannot be said that there was ever an occasion when we did not need to use this weapon, but we needed it as much as ever before at the end of July, 1943, and it was just at that time that the Air Ministry after I had urged the use of this weapon at repeated intervals for many months, decided that it was now possible to accept the risk of the enemy using the same weapon against our own defences. The strips of paper-they were given the code name "Window" - were dropped for the first time on the night of July 24th-25th. The target was Hamburg, beyond Oboe range.
No air raid ever known before had been so terrible as that which Hamburg had endured; the second largest city in Germany, with a population of nearly 2,000,000, had been wiped out in three nights. And at the same time the whole system of air defence, carefully built up, at the expense of all the other battle fronts in which the Germans were fighting, over a period of years, had been thrown into utter confusion; the night fighters, it appeared, would in future be powerless to detect the bombers in the dark, and the guns and searchlights would be altogether inefficient. The first type of Window used by Bomber Command in the attacks on Hamburg was designed to confuse the enemy's Wurzburgs, used both for ground control of fighters and for gun laying, and we knew at once that it had been successful in this. But the enemy also knew what we discovered later, that Window seriously interfered with the night fighters' airborne radar as well.