In December 1941 Spencer put the Air Car into storage and joined the war effort as a test pilot for the Republic Aircraft Corporation. By 1943 he had flight tested 134 of the company's P-47 Thunderbolts.
In April 1943 Spencer left Republic Aircraft to join the Mills Novelty Company of Chicago, Illinois who wanted to use his Air Car to promote their company. Spencer used the company's wood forming equipment to build a new egg-shaped cabin for the Air Car and began demonstrating the aircraft to his former employers, Republic Aircraft. Seeing the potential of the Air Car as the perfect sports plane for pilots returning from the war, Republic purchased the rights to the Air Car in December 1943 and immediately began development of an all-metal version designated the Model RC-1 Thunderbolt Amphibian. On November 30 1944 the first RC-1 Thunderbolt Amphibian, registered NX41816, made its first flight with Spencer at the controls.
The aircraft was displayed in St Louis, Missouri in December and by the end of 1944 Republic had received 1,972 civilian orders for the $3,500 airplane. The aircraft was also demonstrated to the U.S. Navy and Army Air Corps. Both services were impressed with the design and on February 19 1945 the Navy granted Republic Aviation the rights to use the name Seabee for the civilian version. The Army placed a large order for the aircraft, to be used for air-sea rescue operations under the designation OA-15. In September 1945, following VJ Day, both the Army and Navy canceled their orders, which by that time totaled over $20,000,000. The OA-15 Seabee was the last United States Army Air Corps aircraft to use the OA designation, which was dropped when the US Air Force was formed as a separate military branch in 1947.
Military operators included the Israeli Air Force, Paraguayan Navy, the United States Army Air Forces and South Vietnamese Air Force.
In March, 1946 the first production RC-3 Seabee was completed (NC87457, formerly NX87457), and on July 25 1946 the first Seabee (NC87463, production #13) was delivered at the Republic factory to J.G. (Tex) Rankin of Rankin Aviation Industries of Tulare, California.
In the late 1940s aircraft manufacturers hoped that military pilots returning from the war would want to continue flying civilian aircraft for pleasure and sport. This never occurred to the extent the companies imagined, as most wartime pilots returned home never to fly again. As a result many small, optimistic aircraft companies appeared then quickly disappeared in the immediate postwar years.
On October 4 1947 Republic Aviation Corp. announced that it was discontinuing production of the RC-3 Seabee amphibian for the personal plane market. Republic President Mundy I. Peale stated: "Due to the need of all Republic's production facilities for the manufacture of other types of airplanes, the company has decided to discontinue production of the Seabee". Actually, by summer 1947 the Seabee sales had almost stalled and since June 1947 the production had been put on hold, awaiting further sales. By the end of production a respectable 1,060 Seabees had been built. Though this was far from the 5,000 Seabees per year Republic had hoped to sell, it still represented a significant number of airplanes compared to other struggling aircraft companies of the same era. Only Piper, with their cheap, long lived Cub and Super Cub, Beech's popular Bonanza, and Cessna's early 140s and 150s would sell in numbers greater than the Seabee. This was due in no small part to the very low price of the Seabee. During production, however, the price of the Seabee would rise from its original $3,500 to $4,495 effective July 15, 1946, and to a final price of $6000 effective November 15, 1946. This still represented a remarkable bargain for a four seat all metal amphibian. Republic sold its last new Seabee in 1948. By that time the demand for civilian aircraft had shown itself to be far less than anticipated, and Republic turned its attention back to military contracts, developing the successful F-84 Thunderjet, which was built on the same assembly lines formerly used to build the Seabee.
In the late 1940s and throughout the 1950s, the Seabee became one of the most popular bush planes and air ambulances in countries like Canada, Norway, Sweden as well as the USA. Many life-saving missions were flown by hero Seabee pilots rescuing seriously ill people from remote islands and vast wilderness, as well as Seabee pilots making "impossible" takeoffs and landings from small lakes to deliver supplies to hunters and trappers.
In 2006, over 250 Seabees are still registered and flying, a number that is increasing yearly as new aircraft are assembled from parts and wrecks. A few Seabees are even still operating commercially as bush planes and air taxis. In the history of aviation few aircraft have had longer, or more successful careers than the Seabee.
Modifications to increase the engine size, length of wing, revise the internal structure to provide additional cargo space, modify controls, trim activation, adding a heater, landing light and many others have been available since the early 1970's.