For a full history of "stealth" for combat aircraft you have to go back to World War II...as TE referenced...and perhaps further, but I know of two US aircraft that were specifically set up for "stealth" during the war.
You have to remember that radar in World War II was much less sophisticated and the term stealth did not apply to radar back then, it applied to the ability to simply observe an aircraft.
So the US developed a couple of aircraft to be stealthy, meaning very low observable.
One were the PBY Black CAT flying boats. These were special Catalina's that were painted all black, had flame dampeners over their exhaust ports, had special radar altimeters, and flew without lights. They were used at night to raid Japanese bases and shipping and they were very successful. The first squadron operated off of Guadalcanal after it was pacified in 1942. By the end of the war, 14 squadrons had been assigned to the Black Cat mission.
Another was the P-61 Black widow, which was a night fighter for the US. It too was set up to intercept at night and was used for raid missions as well as intercepts of Japanese night bombing attempts. It was the first aircraft that used radar o guide itself to its aircraft targets at night.
After World War II, as radars became more and more powerful, and as US bombers became larger and larger, attempts started to be made to find a way to overcome the radar of foes.
The first true success at stealth in terms of defeating radar occurred rather accidentally. This was with the XB-49 flying wing. Three aircraft were built by refitting XB-35 flying wing aircraft that had been built after 1946 by Grumman. Those aircraft were prop driven and thirteen were built. But with the advent and success of the B-36, the program was cancelled. Along about the time the B-36 started getting its jet engines, the US Air Force called for replacing the propellers on some of the XB-36 with Jet Engines and testing them. This became the XB-49. In 1948 one of the aircraft flew out into the Pacific on a test. Upon returning, when it was returning directly toward the radar that was tracking it...the radar site lost the aircraft and did not find it again until it flew over the base preparing to land.
But the XB-49 was difficult to fly and very unstable. After one crashed, the program was cancelled. Interestingly enough, Grumman later built the first true stealth bomber, the B-2, and used data from the XB-49 to help design it. In fact, the wingspan of the B-2 is the same as that of the XB-49.
In the mean time, the need to defeat enemy radar continued. This began with jamming (which continues to this day) and more and more sophisticated electronic warfare and jamming devices were developed and deployed on the various bombers...the B-46, B-52, B-58 and so forth. Today's bombers use ever more capable and sophisticated jamming equipment.
In addition to jamming, in the 1950s, the US developed sophisticated drones that would mimic the radar signature of the aircraft that carried them and thus fool enemy radar into shooting at the wrong targets. This was a form of stealth and the most prolific of these at the time was the ADM-20 Quail. They were made to launch from the bomb bay of the B-52 and fill the air with false signals. Hundreds of them were developed and deployed, first flying in 1960, and being used until 1978.
The US also needed to penetrate enemy defenses for surveillance missions. Initially this requirement is what developed the U-2 program, the first of which went into service in 1957. The U-2 was not fast, and it was not agile. It flew very high, and it also was the first production aircraft to make use of radar absorbent paint on its surfaces. That technology has continued to develop since, but at the time although the material was successful at first, it was only effective against certain amounts of power. As the Soviets and others developed more powerful and more sophisticated radars, they were able to detect and lock onto the U-2...and ultimately shoot it down.
Once the Soviets proved their ability to detect and shoot down the U-2, the US moved on to other technologies to avoid radar detection and lock on. Once of these was the improvement of the U-2. This ultimately resulted in the TR-1 versions (later renamed the U-2S) which continued to be used until just recently.
But it also led the US to develop even more sophisticated aircraft that combined low observability, altitude, and speed. This type of requirement initially came from the reYF-12A interceptor. But that aircraft was not deemed very effective in the interceptor role. But it was ultimately developed into the surveillance role, with more stealthy qualities and became the SR-71 spy plane. In this role, the aircraft excelled.
The SR-71 was the first operational aircraft to use radar absorbing material/structure in its airframe. It was used along the leading edges and on the flat control surfaces. While not having the technology to be as effective against radar as later technologies, the material reduced its signature well enough to allow its other qualities...it's high altitude (in excess of 80,000 feet) and high speed (in excess of Mach 3), to allow it to complete its mission without being defeated by opposition radar and missile sites. The first SR-71 was introduced in 1966. It flew until the 1999-2000 time frame. Not a single SR-71 was ever shot down or lost to opposition efforts.
Ultimately, the through this 40 year process, the US developed its understanding of and use of stealth technologies to a point where it was ready to try and defeat contemporary radars completely. This ledd to the programs (like Tacit Blue) which culminated in the F-117 Night Hawk stealth fighter. This in turn led to the B-2, and ultimately the F-22 aircraft (probably with an aircraft, sometimes called the Aurora, which may have replaced the SR-71 in service in the late 1990s). But that brings us to the point where TE started the thread.
Although thjis is not exhaustive by any means, I just wanted to give a flavor of some of the development of "stealth" that led to where we are today
or something), would be used only in quotes.
