From 1934 to 1936, experiments by the Naval Research Laboratory, NRL, were being carried out by Robert M. Page. In 1935, he and a small group of scientists at NRL began testing a 60 MHz pulsed radar to detect aircraft. However, pulses from the high powered transmitter caused "ringing" in the receivers, which swamped the echo signals returning from objects nearby.
By 1936, NRL engineers had built a 28 MHz pulsed radar which detected aircraft 10 miles away. By the end of 1936, a new radar operating at 80 MHz was detecting aircraft 38 miles away. And in Dec. 1938, a 200 MHz XAF radar was tested on the USS New York and it detected aircraft up to 100 miles away. The Radio Corporation of America, RCA, worked closely with NRL. Hollmann's patent disclosures filed in the US for such systems as pulsed radar circuits, ring oscillator multiple push-pull tuned-grid tuned-anode, split-anode magnetron, Barkhausen-Kuerz oscillator and antenna duplexer were used.
The new model CXAM radar, developed after the XAF unit, worked on 400 MHz (l=75 cm), was put into production. RCA built and delivered 20 CXAM sets in 1940. These sets were installed on the battleships California, Texas, Pennsylvania, West Virginia, North Carolina and Washington, on aircraft carriers Yorktown, Lexington, Saratoga, Ranger, Enterprise and Wasp, on 5 heavy cruisers, 2 light cruisers, and the seaplane tender Curtis. The CXAM performed well during WWII.

The United States Army Signal Corps also started developing radar as early as 1930. In 1935, tests on microwave propagation tests using Hollmann built valves, RCA magnetron operating at 9 cm, RCA acorn valves were performed. In 1937 the test radar unit was demonstrated. Based on this test unit, in 1940, the SCR-270 became available for coastal defense and it was first deployed in Panama in the Fall of 1940 as an early warning for the Air Corps, Pursuit Squadron. This unit operated on a frequency of 205 MHz (l=1.5 m) and had a range of 23 miles, had an angular accuracy of 1 deg. 18 units were built by the Army Signal Corps Laboratory for training purposes. By June of 1941 a total of 85 sets had been delivered by Western Electric. A total of 794 were produced between 1939 and 1944.

On the left is a SCR-271 3 m air warning radar at New Caledonia in May 1943. The SCR-271 was an early version of the SRC-270.

In October 18, 1940 the Radiation Laboratory at the Massachusetts Institute of Technology with L.A Du Bridge as its technical director was set up with the prime purpose of developing radar for the war effort. It had three primary goals: 
1. Develop a 10 cm Aircraft Interception (AI) radar. 
2. Develop a gun laying radar. 
3. Aircraft navigation. In March 1941, a 10 cm radar was tested on a B-18 bomber. 
In its 6 years of existence, over 2.1 billion dollars were spent on the development of radar. This was about as much as was spent on the development of the atom bomb.