What do you think is the purpose of a bomber?
To deliver significant amount of firepower to a distant location and return without the need to stop and/or refuel on a frequent basis. A single bomber can destroy as many targets as possible in a single sortie - a task that would require multiple sorties from combat aircraft to fulfill otherwise (risk factor notwithstanding). Bombers are ideal platforms to soften A2/AD arrangements in a span of few hours.
B-21 Raider represent next generation of stealth (ELO class); current crop of A2/AD systems do not offer a solution for VLO class aircraft, let alone ELO class.
The (former) Yugoslavia had acquired all manner of sophisticated A2/AD equipment of Russian and French origin in the 1990s including Russian [anti-stealth] types and its terrain compounded the challenge yet further. I can provide further breakdown of the equipment if necessary.
PRESS RELEASE
before Operation Allied Force (1999):
WASHINGTON -- An allied attack on Serbia would begin with a barrage of sea-launched cruise missiles aimed at destroying the sophisticated air defenses that pepper the country's craggy hills, Pentagon and NATO officials say.
The missiles, each armed with a 1,000-pound warhead, are to pave the way for attacks by hundreds of NATO warplanes. Even so, officials expect casualties because of the anti-aircraft system, which is far more advanced than the ones encountered in the NATO bombing campaign in Bosnia in 1995 or in the continuing raids over the Iraqi no-fly zones.
To cripple the system, allied warships in the Adriatic Sea would unleash hundreds of missiles, possibly followed by attacks by B-52 bombers firing cruise missiles carrying 2,000- and 3,000-pound warheads.
100 SAM sites
Slobodan Milosevic of Yugoslavia has amassed an impressive array of 1,850 anti-aircraft artillery pieces and more than 100 surface-to-air missile (SAM) sites concealed in Serbia's forested and mountainous terrain, which resembles West Virginia's.
Serbia's terrain would be far more dangerous than the "tabletop" desert of Iraq, where air defenses can easily be spotted and destroyed. No allied planes have been hit in the patrols of the Iraqi no-fly zones.
"You can hide [artillery and missiles] behind a hill and stick an antenna up," said a NATO official, explaining that even powerful cruise missiles will be hard-pressed to destroy Serbian anti-aircraft sites that have been dug in.
Moreover, Milosevic has put together heavier concentrations of air defenses than has Iraqi President Saddam Hussein.
Air Force chief's warning
Consequently, Gen. Michael E. Ryan, the Air Force chief of staff, has warned lawmakers to expect casualties among U.S. pilots.
"There is a distinct possibility we will lose aircraft in trying to penetrate those defenses," Ryan told the Senate Armed Services Committee last week. "This is a very substantive air-defense capability."
A Soviet-made surface-to-air missile brought down the F-16 piloted by Air Force Capt. Scott O'Grady over Bosnia in 1995. Several days later, O'Grady was recovered in a helicopter rescue by U.S. Marines.
Despite such sophisticated weaponry, no casualties occurred in the Bosnia bombing campaign, which some say convinced Milosovic to enter talks that led to the Dayton peace accord.
Ryan said the Yugoslav defenses are "more modern, put together in a better way, strung together in-depth" than the ones faced by allied pilots in neighboring Bosnia.
https://www.baltimoresun.com/news/bs-xpm-1999-03-23-9903230139-story.html
NATO knew and understood WHAT KIND OF DEFENSES they were up against in then Yugoslavia.
Yugoslavian A2/AD defenses were able to score hits on LO class targets such as Tomahawk cruise missiles and also on one F-117A Nighthawk (only a dozen created).
But they were unable to touch B-2 Spirit bombers which devastated much of Yugoslavia
in just 50 sorties; traces of such destruction can be found even today after so many years of repairs.
Relatively inferior F-117A Nighthawk strike aircraft pulled off
over 800 sorties over Yugoslavia in total. The one was that was shot down happened in the beginning when Yugoslavian A2/AD defenses were FRESH and in good form.
Operation Allied Force turned into a nightmare for Yugoslavia instead. Volley after volley of Tomahawk cruise missiles came first with EA-6B Prowlers and F-117A Nighthawks penetrating Yugoslavian A2/AD arrangements from the above. When one of the F-117A was lost in a sortie, then B-2 Spirit bombers came and devastated much of Yugoslavia in turn.
No country has managed to produce something on the lines of B-2 Spirit thus far - a 1980s American bomber design
but SO FAR AHEAD of anything in the domain of aviation at the time that it fascinate people to this day. B-21 Raider is EVOLUTION of the same design concept.
Do not mistake these birds for some drones. WE are looking at things beyond comprehension of average minds in B-2 Spirit and B-21 Raider by extension. These represent the epitome of American aviation capabilities and expertise. It is obvious that greatest of minds are sought for these constructs and much of their respective capabilities remain shrouded in secrecy.