Does bleaching or milling flour remove vitamins?

Question

Does bleaching or milling flour remove vitamins? Is this why some flours are enriched, even non-bleached ones, with, e.g., "niacin, reduced iron, thiamine mononitrate (vitamin B1), riboflavin (vitamin B2), folic acid" (source)?

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for 100 g of each flour:

Flour	Vitamin B3 (niacin)	Iron (Fe)	Vitamin B1 (thiamine)	Vitamin B2 (riboflavin)
Flour, bleached, enriched, all-purpose, wheat	6.740 mg, 42%	5.62 mg, 31%	0.939 mg, 78%	0.443 mg, 34%
Flour, unbleached, enriched, all-purpose, wheat	7.070 mg, 44%	5.41 mg, 30%	1.050 mg, 88%	0.467 mg, 36%
Flour, unbleached, unenriched, all-purpose, wheat	1.590 mg, 10%	1.18 mg, 7%	0.298 mg, 25%	0.000 mg, 0%
Flour, unenriched, whole wheat	5.550 mg, 35%	3.86 mg, 21%	0.504 mg, 42%	0.128 mg, 10%
Wheat flour, unenriched, all-purpose, white	1.250 mg, 8%	1.17 mg, 6%	0.120 mg, 10%	0.040 mg, 3%

As the data show, extra processing (bleaching, refining for white flour) removes a substantial amount of vitamins (whole wheat has roughly 3× the amount of vitamins compared to unenriched white or all-purpose!).

Answer 1

It's not the bleaching, but the milling which causes this loss of vitamins.

The lack of certain vitamins in white flour led to some serious diseases. Enrichment began to counter these issues.

Flour enrichment can provide an important source of iron in the diet. In the U.S., flour enrichment was established in the 1930’s as the popularity of white flour increased. Health concerns about the rise in cases of diseases such as beriberi and pellagra led to an examination of white flour. Both beriberi and pellagra are caused by vitamin and mineral deficiencies. The addition of B vitamins and iron to white flour eliminated the diseases beriberi and pellagra.

The initial enrichment of white flour called for niacin, thiamin, riboflavin and iron. In the 1990’s folic acid was added in order to prevent the risk of coronary heart disease and birth defects such as spina bifida.

from https://bakerpedia.com/processes/flour-enrichment/

Answer 2

Yes, refined flour has substantial loss of nutrients:

^{Whole Grain Council}