India's first ever official airmail flight 1911, Allahabad, UP - world's first ''Aerial Post''

First aerial post, Allahabad, India.18 Feb 1911postalmuseum.si.edu
Though air-plane industry, research and development were mainly taking place in the US and Europe, it was in India, believe it or not the first official airmail flown by air plane took place  deep in the United Provinces.

Walter Windham, an Englishman in 1910 was invited by the United Provinces (now part of Uttar Pradesh) to take part in their annual cultural and commercial exposition in Allahabad  to be held in Feb. 1911. They requested him to bring his planes and spares, etc to conduct demonstration flights to create an interest among the Indians in the art of flying - a  new venture in those days. 
Tndia. 1911First aerial post, postalmuseum.si.edu
Special cancel used only one day, India, Feb 18, 1911First aerial post, postalmuseum.si.edu
Experienced French pilot Henri  Pequet was  also in India to hold flying demonstration flights for the United Provinces Exhibition in Allahabad,  During his stay there in Allahabad  for the commercial exposition, one  Rev. W.E.S. Holland, a chaplain of the Holy Trinity Church, Allahabad,  contacted Windham and requested him to help him raise funds for a  youth hostel project. It dawned upon Windham to introduce  aerial post  in India as there was no such facility available and the colonial government and the public were dependent on the overland postal services which were slow and not attractive.  The chaplain  accepted the proposal and Windham approached the postal deptt. Geoffrey  Clarke, Postmaster General of the United provinces and the Director of the Post Office, India granted approval  for mail to be officially received and cancelled  prior to transport by one of Windham's planes.  Postal officials asked Windham to design the cancel. Most mail has a magenta cancellation, but a few examples exist with black ink. The regular postage rate required an additional surcharge as a donation for the Church Hostel Building.

The public was invited to deliver stamped and addressed mail to the Chaplain of the Trinity Church enclosing six pence or annas with each letter to be posted by air. People could also get the mail sent by air with the help of a postal official in the hanger on the parade grounds.The mail flight was scheduled to take place on 18 February 1911, two days earlier. As it happened to be a sensational event, thousands of people saw Pegquet take off with mails.  French pilot Henri Pequet (1888-1974) carried the first official mail flown by an air plane. He carried a sack with about 6,500 plus cards and letters on his Humber biplane. The plane flew a distance of five miles, from an Allahabad polo field, over the Yamuna River, to a place called Naini and took 13 minutes to land there.. All mail received a special cancel depicting an airplane, mountains, and “First Aerial Post, 1911, U. P. Exhibition Allahabad.”.The postal official on the outskirts of Allahabad received the mails, sorted them out for surface transport to destinations world over. 
gandhistampsclub.blogspot.com
Tit-bits:
01. The postal mark used on the air mails was  distinctive and made in the postal works in Aligarh, UP.  

02, The event also marked the first ever flight in India and was held during the Maha Kumbh in 1911.

03. The letters bore an official frank "First Aerial Post, U.P. Exhibition, Allahabad. 1911"

04, On the basis of this successful experiment in India, the world's first scheduled airmail post service took place in the United Kingdom between the London suburb of Hendon, North London and the Postmaster General's office in Windsor, Berkshire, on September 9, 1911, as part of the celebrations for King George V's coronation

05, The first official U.S. airmail delivery took place on August 17, 1859. On that day, veteran balloonist John Wise (1808-1879) carried 123 letters and twenty-three circulars from Lafayette to Crawfordsville, Indiana, a distance of thirty miles, in his balloon Jupiter.
https://postalmuseum.si.edu/collections/object-spotlight/india-air-mail.html
https://www.indiatoday.in/education-today/gk-current-affairs/story/worlds-first-official-airmail-309327-2016-02-18
http://www.istampgallery.com/first-aerial-post/