There are hundreds of public domain images in the Flickr Commons contributed by the National Archives of the United Kingdom. There are three collections: Africa Through a Lens; Artwork, Artifacts and Documents; and Photography.
Today’s images are from sets within the Photography Collection.

German Graf Zeppelin flies over St. Paul's Cathedral while on a press visit to London. 1930. http://flic.kr/p/8JJzmx

A British made Cierva Autogiro flies over Manhattan Island. The brainchild of Spanish aeronautical engineer Juan de la Cierva, the autogyro was developed in the early 1920s. http://flic.kr/p/74b56X

Hiram Maxim's Glider
The Anglo-American inventor of the machine gun, Sir Hiram Maxim experimented unsuccessfully with powered flight during much of the 1890s. In 1911 he established a company to develop military aircraft with Louis Blériot, the first man to fly across the English Channel and engineer Claude Grahame White, one of Britain's first qualified pilots. c.1915
http://flic.kr/p/7hPrmA

Fighter squadron in formation. Photograph from the log book of RAF Squadron Leader B.J.E. Lane. 1940-2. http://flic.kr/p/8t3C1R

Photograph from the Trench Warfare Section of the Ministry of Munitions showing a man wearing breathing apparatus against gas attack. WWI. http://flic.kr/p/5Rc3AV

In the early 20th century sailing ships would compete to be the first to bring a grain cargo from Australia to the UK. Built in 1911 S.V. Passat was the winner of the final race in 1949/50, reaching Penarth, Wales in four months. Its cargo of wheat was destined for the mills of Methodist mill owner and film magnate J. Arthur Rank. http://flic.kr/p/8JJk6r

'With Captain Scott to the South Pole (British Antarctic Expedition)'. Steam Yacht 'Terra Nova' with dogs on Ice at side.'
"This image shows a single frame from the very short (3-4 frame) sections of nitrate film stock accessioned at The National Archives from Herbert Ponting's footage of the Antarctic." c.1911.
http://flic.kr/p/9EMMDe