The National Loaf was a government regulated and mandated loaf of bread sold nationally in Britain from 1942 onwards as part of the home front effort during World War II (1936 - 1945.)
By the outbreak of the war in September 1939, Britain, with a population of around 50 million, was importing 70% of its grains, with the remainder grown at home. The bulk of that imported grain was, as it is now, wheat shipped from Canada across the North Atlantic. Though the ships travelled in protected convoys, they were vulnerable to attack and sinking by German submarines. To give everyone even just an small, inadequate amount of bread daily such as 1/2 oz, planners calculated that 30 ships were required to import in 250,000 tons of wheat a year. That was wheat for bread alone, not counting all the wheat required for other food products.
This shipping cost scarce money and resources that could be used elsewhere in the war effort: the less shipping space required for wheat, the more space was available for war materials.
Notwithstanding that, though other food stuffs had been rationed since January 1940, the British government did not want to ration wheat or bread for as long as could possibly be avoided.
The Ministry of Food was therefore charged with reducing the amount of imported wheat required and making the most of what did arrive, while continuing to keep bread freely available and ensuring that the population was receiving optimal nutrition value from their food.
The solution they arrived at was to make the wheat they did import go further by the creation of "National Wheatmeal Flour" or "National Flour" in the spring of 1942.
National Flour"To cope with the reductions in the amount of wheat imported, more flour was extracted from the grain available. The extraction rate was raised to around eighty-five percent giving the population the nourishing wholemeal National Loaf (with a high vitamin B content), although many people did not find its greyish colour appealing." 
"National wheatmeal flour" was unbleached flour of 85% extraction from hulled wheat grains. The flour had in it the starchy endosperm, the wheat germ, and the bran, with the coarser bran extracted out. 85% means that out of 100 kg of wheat grains, you would get 85 kg of flour. White flour is generally around 70% extraction, and would get you 70kg. The higher 85% extraction rate gets you an extra 15kg of flour from that wheat.
National Flour was consequently similar to wholemeal (aka wholewheat) flour, but with some of the coarser bran removed, which in wholewheat flour is left in. For making bread, it usually had some white flour mixed in as well.
The initial extraction rate was 85%, but it varied over the years. Furthermore, two extraction rates were often referred to: the actual initial extraction rate of the flour from the wheat, and the effective extraction rate after that flour was mixed with some white flour for bread use:
Figures on what the extraction rate was at any one time vary based on the source. It is likely that some sources look at effective extraction rates: "In 1944, the 85% required extraction rate was lowered to 82.5 %, and then later that year, to 80%. 
White flour was still being produced and imported during this period, but it could only be obtained by food manufacturers for items such as biscuits, cakes, etc, or for mixing in small quantities into 85% extraction flour to make National Flour. Flour milled in Britain, whether from domestically-grown or imported wheat, was 80% extraction (by 1945.) Imported already-milled flour was 75% extraction. To make National Flour, the imported flour was mixed in with domestic flour at a rate of about 15% imported, 85% domestic. In Scotland, for some varieties of national bread such as batch bread, etc, bakers were allowed to mix in up to an extra 12 1/2 % of imported flour.  
Overall, allowing that white flour was still being produced, Ministry of Food officials estimated that changing that extraction rate for the bulk of bread production requirements reduced the amount of wheat Britain required by about 10%.
In addition to its higher extraction rate, National Flour was also a fortified flour with calcium and other vitamins added to it.
The decision to add calcium was opposed by some, but the Government based its decision on research done by Robert McCance and Elsie Widdowson, who recommended 120mg calcium carbonate be added to each 100g of 85% extraction National Wheatmeal flour . In practice, it was added at the rate of 14oz per 280 pounds of flour. The purpose of the calcium was to offset the higher rate of phytic acid in the higher extraction flour which impedes calcium absorption and can therefore lead to rickets in children (as happened in Dublin, where the calcium supplement was not added.)
The calcium was phased in gradually by various flour producers:
The calcium carbonate enhancement was added to the 85% extraction flour and to white flour, but it was decided not to add it to actual wholemeal (wholewheat) flour, as a result of some opponents who felt that calcium posed dangers: "Wholemeal bread however, which because of the phytic acid content was most in need of fortification, was not fortified in deference to the pure food enthusiasts who bitterly opposed fortification as soon as it was proposed." 
The enrichment thus looked after the calcium issues that higher-grain bread can cause, but medical researchers began to express concern about the impact of the phytic acid in the flour inhibiting the absorption of iron. Even if the bread were fortified with additional iron, it still didn't get absorbed by the body. 
The National LoafNational Flour was used to make the National Loaf, a yeast-risen, bakery-made bread for daily consumption.
A ban on commercial, pure white bread production came into effect on 6 April 1942 . The regulations stated that 75% of the wheat flour in a loaf of bread had to be of 85% extraction (the rest of the wheat flour could be regular white wheat flour), that the bread had to be sold unwrapped, and unsliced, that the bread could only be sold the day after it was made, not on the day of, and that the official legal size of a loaf of bread was reduced from 16oz to 14 oz.
The definition of the National Loaf was adjusted over the years.
In September 1942, William Mabane, Parliamentary Secretary to the Ministry of Food, said that the government would consider diluting the wheat flour with oatmeal or potato flour.  And indeed, by 1943, up to 10% of the ingredients could be barley flour, or a mixture of barley flour and oat flour. 
Potato flour was also allowed by the summer of 1943:
The National Loaf also had quite a high amount of salt in it, to make it keep longer.
85% extraction rate flour is used today in France to make the famous, and expensive, Pain Poilâne. Be that as it may, the British government had to churn out a good deal of propaganda to get people to accept the National Loaf. People loathed it; they called it "Hitler's Secret Weapon." The bread looked grey and unappealing, and it was coarse, with a texture like sawdust, and dry (and stale by the time they were allowed to buy it, 1 day after baking), and had a very tough crust to boot.
Countering this were rumours that the bread had certain aphrodisiac qualities. Some sources feel that the rumours were deliberately put out by Lord Woolton, head of the Ministry of Food. The "basis" of the rumours were that the loaves were rich in wheat germ, which is rich is Vitamin E, and that Vitamin E increases fertility.
Titillating or not, many people still said loudly and publicly that the National Loaf was indigestible. But by 1945, bread consumption in Britain had gone up 20% over 1939, even though only the National Loaf was available, so someone was eating it.
Bread was never rationed in Britain during the war, though ironically it was for a short period after the war, from 21 July 1946 to July 1948. Some feel that the reason Prime Minister Attlee's government introduced bread rationing was more political than out of actual necessity. 
In 1950, sliced, wrapped white loaves were allowed to be sold again, even though a few people felt that for population health reasons, the government should keep the National Loaf as the only legal bread. They feared that allowed a choice, people would go back to white bread -- which in fact they did.
In 1956, the National Loaf was discontinued,
Developers of The National LoafAmong the people who worked on developing specifications for the National Loaf were Harriette Chick, Elsie M. Widdowson, and Robert McCance.
Harriette Chick (6 January 1875 to 9 July 1977) was a medical research scientist who worked in nutrition and public health. In 1905, she became one of the first paid women scientists in the UK. Her involvement in the National Loaf was to make sure that it delivered maximum nutrition. After the war, she was made a Dame of the British Empire in 1949 for her contribution to the National Loaf. Chick never married.  
Dr Elsie M. Widdowson (22 October 1906–14 June 2000) worked as a team with Dr. Robert Alexander McCance (9 December 1898 - 5 March 1993) on the nutritional aspects, particularly avoidance of calcium deficiency. In 1979 she was made Commander of the Order of the British Empire; in 1993, she was made a Companion of Honour by the Queen. 
National Loaf Recipe
Source: Zana Richardson, Imperial War Museum
For 10 loaves (for 1 loaf, divide by 10)
Wholemeal flour - 5220g
Potato Flour - 1740g
Salt Sea Fine - 140g
Tap Water - 4740ML
Vitamin C - 6g
Yeast - 210g fresh or 84g active dry
1. Mix all ingredients in a mixer for 3 to 5 minutes.
2. Place dough in lightly oiled container, let rest for 45 minutes
3. Knock back and let rest for another 45 minutes
4. Scale at 1kg, first shape (round)
5. Rest 10-15 minutes, then second shape
6. Place bread in oiled baking tins, prove for 45-60 min at 28-32c
7. Bake at 208c top 204c bottom, with 5 sec steam. Open vent after 25 min, bake for a further 25 minutes. Remove from tins immediately and cool on a rack
Literature & Lore
As one who analyses practically every boat-load of wheat arriving in this country, I must point out that the statement made in Sir Ernest Graham-Little's letter is completely incorrect. The germ is not removed in the country of origin but arrives here as a constituent part of whole wheat.
Secondly, Dr. Frewen Moor (p. 531) asks what goods are made from flour in the milling of which it is permissible to extract germ -- i.e., flour destined for manufacturing purposes. Such flour (known officially as "M" flour) is mainly used for self-raising flour or for making biscuits and confectionery goods. These goods are usually more alkaline than yeast-made bread, and in consequence of the pH being 7.0 or over the B1 present is largely destroyed in the baking process. Hence the decision of the Ministry of Food to allow germ to be extracted in such cases, which represent only a small percentage of the total flour used." -- D.W. Kent-Jones, Ealing W5. "Flour in the Loaf." Br Med J 1945;1:644 doi:10.1136/bmj.1.4400.644 (Published 5 May 1945) Page 644.